NOVELTY - Use of composition comprising at least one ionic liquid which is lipophilic and non-miscible with water, and at least one enzyme is claimed, for esterification and/or transesterification of substrate with at least one alcohol, where the substrate comprises oils, fats, fatty acids, or their mixture, and the ionic liquid, substrate, and alcohol form single homogenous liquid phase at a temperature at which the esterification and/or transesterification process is performed.    USE - The composition is useful for esterification and/or transesterification of substrate with alcohol (claimed), for producing biodiesel.    ADVANTAGE - The biodiesel can be produced using recyclable solvent with low environmental impact, and improved reaction kinetic without catalyst poisoning, and the reaction products can be separated easily. The different types of oils, fats or fatty acids can be used in the same process. The presence of water and/or free carboxylic acids in raw oil or fat does not severely impair the process.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method for esterification and/or transesterification of substrate into fatty acid alkyl esters, involves initialization step of combining at least one substrate, at least one alcohol, and at least one enzyme in at least one ionic liquid, where the alcohol and substrate are in appropriate amounts to form fatty acid alkyl esters, and the ionic liquid is liquid at a temperature at which the process is performed, recovery step of recovering the fatty acid alkyl esters formed in the esterification and/or transesterification reaction, and recovering the glycerol formed in the esterification and/or transesterification reaction, and restoration step of recovering and purifying the composition of ionic liquid and enzymes; and    (2) reaction mixture comprising enzyme and single homogenous liquid phase at a temperature, preferably room temperature, at which the process is performed, containing at least ionic liquid, at least substrate, and at least alcohol.