NOVELTY - An ionic liquid containing an ammonium cation, is new, where the ionic liquid contains water (less than or equal to 1 mass%).    USE - As liquid carrier, and solvent for at least one species; as a reaction medium for a chemical or biochemical reaction such as enzyme-catalysed (all claimed). The ionic liquid useful as a solvent for numerous materials e.g. biopolymers (such as proteins and polysaccharides), natural products (such as terpenes, alkaloids and oils) and minerals, chitin and other aminopolysaccharides; as a matrix in matrix-assisted laser desorption/ionisation mass spectrometry; as a solvent for a solvent extraction process; as a carrier for use in chromatography including gas chromatography; as a hydraulic fluid or lubricant; as a coolant or heat exchange medium; as a cleaning agent; as a wetting agent; as a biocide; as a preservative or fixative; and useful in liquefaction, nuclear fuel reprocessing, fuel cells, electrochemical applications, pervaporation, drug delivery, adhesives and sensors.    ADVANTAGE - The ionic liquid is used as an alternative to the usual solvents, with retention of enzyme activity. It provides a stable reaction medium with significant advantages over conventional solvents, in particular higher achievable substrate concentrations, increased biomolecular stability and eco-compatibility. The ionic liquids offer higher levels of enzyme activity without risk of the solvent participating in the reaction; and also exhibit excellent electrical and thermal conductivity whilst being non-flammable, recyclable and generally of low toxicity.    DETAILED DESCRIPTION - An ionic liquid containing ammonium cation of formula R1R2R3N+-OR4 (I) is new. The ionic liquid contains water (less than or equal to 1 mass%).    R1 - R4=H or hydrocarbyl.    INDEPENDENT CLAIMS are included for the following:    (1) a composition comprising the ionic liquid in combination with an enzyme and optionally also a substrate for the enzyme;    (2) preparation of the ionic liquid.