Many deep eutectic solvents (DES's) are mixtures of ammonium salts and hydrogen-bond donors; e.g., a 1:2 mixture of choline chloride and urea. Like room temperature ionic liquids, DES's are polar, viscous solvents with low vapor pressure and flammability. However, synthesis of DES's is simpler - requiring only mixing of components - and the components are non-toxic and approximately ten-fold less expensive than the components for ionic liquids. Even though components of DES's can include protein denaturants like urea, we found that immobilized Candida antarctica lipase B retains activity in a wide range of DES's. Rates of transesterification, aminolysis of esters, perhydrolysis were similar to those in organic solvents and several-fold faster than those in ionic liquids. In a few cases, side reactions with the components of the DES occurred. DES's containing sugars were highly viscous and required temperatures ≥60 °C to permit stirring.