NOVELTY - Separating 2C saturated and/or 3C, and/or 4C saturated and/or unsaturated mono carboxylic acids and/or aldehydes from aqueous solutions by extraction where the improvement comprises using as the extractant liquefied propylene and/or propane.    USE - For separating 2C saturated and/or 3C, and/or 4C saturated and/or unsaturated mono carboxylic acids and/or aldehydes from aqueous solutions, where the carboxylic acids are selected from acrylic acid, acetic acid, propionic acid, butyric acids, and/or methacrylic acid, and the aldehydes are selected from acrolein, acetaldehyde, methacrolein, propionaldehyde, butyraldehyde, crotonaldehyde, and/or 3-butenal (claimed). The aqueous solution comprises waste water.    ADVANTAGE - The method recovers carboxylic acid from wastewater streams, as a concentrate that is compatible with established carboxylic acid purification systems. Further, the method maximizes the water removal from aqueous 2C saturated and/or 3C, and/or 4C saturated and/or unsaturated mono carboxylic acids, especially acrylic acid, to minimize the energy requirements for the down-stream purification of the carboxylic acids. The method achieves desired 2C saturated and/or 3C, and/or 4C saturated and/or unsaturated mono carboxylic acid recovery and concentration at high overall recoveries of such carboxylic acids, i.e. minimizing losses to separation inefficiencies and formation of decomposition/degradation products. In the method, the carboxylic acids, especially acrylic acid (AA), is recovered from dilute aqueous mixtures by liquid-liquid extraction with pressurized liquefied propylene and/or propane, where carboxylic acid is transferred from the aqueous phase into the liquid solvent phase (extract). The carboxylic acid is concentrated from the extract by boiling-off the propylene/propane solvent, which can be condensed and re-used (all, or in part) in the extraction step. The propylene and/or propane used are much more volatile than either water or the propylene-oxidation reaction byproducts that present in the carboxylic acid extracts. They are also much more volatile than alternative carboxylic acid extraction solvents that have been described in the prior art. This high volatility affords several advantages as follows. The solvent can be readily separated from the carboxylic acid and other organics by simple evaporation or flash distillation and the solvent can be condensed and re-used in the extraction without need of further purification. In contrast, some of the alternative solvents described in the prior art require complex fractional distillation or other means to separate and purify the solvent from the carboxylic acid or other reaction byproducts present in the extract. The solvent can be boiled-off from the carboxylic acid at mild or even low/cryogenic temperatures. This minimizes the thermal exposure of the carboxylic acid during the concentration step. This is especially important when the carboxylic acid is AA, which tends to polymerize at high concentrations and/or elevated temperatures. Low-temperatures during concentration reduce the propensity to polymerize during processing, and allowing a reduction in the amount of AA polymerization inhibitors that needs to be added. Low-temperature boil-off of the solvent allows the use of waste heat as the energy source to drive the separation, affording lower operating costs than if a conventional high-quality heat source (e.g. steam, hot oil, or electric heating) was used, as is required with low-volatility alternative solvents. The carboxylic acid can be concentrated without being vaporized and re-condensed, as is required with solvents that are less volatile than carboxylic acid. Propylene and/or propane are used as feed stocks for acrolein and AA manufacture, and typically are supplied to manufacturing facilities in pressurized, liquefied form. Thus, their use as extraction solvents in AA recovery eliminates the need to introduce, store, and handle additional materials, as would be required with solvents that are not native to acrolein and AA manufacture. The carboxylic acid extraction/concentration process using pressurized liquefied propylene/propane has the advantage of mechanical simplicity. Apart from liquid feed pumps and flow control valves, the process can, in principle, be designed with no moving parts, relying on direct liquid-liquid contacting, heat transfer, pressure differences, and gravity flows to effect material transfers and separations. The solvent removed from the extract in the concentration steps can be condensed and re-used for additional extraction, and/or used all or in part for other purposes, e.g. as feedstock for the reactions to produce the carboxylic acids or their precursors. If the condenser is elevated above the top of the extraction section, the (pressurized) condensed solvent can be recycled to the extraction section by gravity flow. The carboxylic acid can be purified from the concentrate by conventional means, e.g. those methods widely practiced in the commercial manufacture of high-purity products such as acrylic acid. Polymerization inhibitors are added to prevent the undesired polymerization of (unsaturated) carboxylic acids (such as AA) that can occur at high liquid concentrations or when subjected to heating.