NOVELTY - Providing a method for removing undesirable components and oxygenated hydrocarbons as well as CO2 and water down to ppm levels in olefin product streams and without removing significant amounts of olefin.    USE - For treating ethylene and/or propylene containing stream, to remove oxygenated contaminants such as acetaldehyde, carbon dioxide and/or water from stream.    ADVANTAGE - The method for treating ethylene and/or propylene containing stream, effectively removes oxygenated hydrocarbons, especially acetaldehyde, carbon dioxide and water down to ppm level in olefin product stream, without removing significant amounts of olefin. Removal of acetaldehyde from the olefin stream can be accomplished by any conventional units capable of removing such by-product to appropriate level. Highly concentrated ethylene and propylene, are produced from the olefin stream. The separated ethylene and propylene streams comprises relatively few hydrocarbon components which causes fouling problems in acid gas treatment systems. The ethylene and propylene streams treated and separated by the treatment method, can be effectively polymerized to form plastic compositions, especially polyolefins such as polyethylene and polypropylene.    DETAILED DESCRIPTION - Treating an ethylene and/or propylene containing stream, involves providing an olefin stream containing ethylene, propylene, C4+ olefins and acetaldehyde, separating the olefin stream into two fractions (I,II), and acid gas treating fraction (I). Fraction (I) comprises at least a majority of ethylene and/or propylene present in the olefin stream. Fraction (II) comprises at least a majority of C4+ olefins and acetaldehyde present in the olefin stream.    INDEPENDENT CLAIMS are also included for:    (1) treating an ethylene and/or propylene containing stream made from an oxygenate to olefin process comprises separating the olefin into first and second fractions, the first fraction containing majority of the ethylene and the second fraction containing majority of the propylene, C4+ olefin and acetaldehyde in the olefin stream, and acid gas treating the first fraction; and    (2) treating an ethylene and/or propylene containing stream made from an oxygenate to olefin process comprises separating the olefin into first and second fractions, the first fraction containing majority of the ethylene and propylene, and the second fraction containing at least majority of the C4+ olefin and acetaldehyde in the olefin stream, and acid gas treating the first fraction.    DESCRIPTION OF DRAWING(S) - The figure shows the flow diagram for separating acetaldehyde and C4+ olefins from ethylene and/or propylene containing stream.    Lines (10, 14, 20, 22, 26, 32, 38, 40, 42, 46, 50, 52, 54)    Quench tower (16)    Bottoms lines (18, 30)    Compressor (24)    Distillation column (28)    Caustic wash column (36)    Dryer system (44)