A process for the transesterification of urethanes comprises reacting a urethane with an alcohol at elevated temperature in the presence of a Lewis base containing catalyst.The process is particularly useful when the alcohol used is a polyol since the product of the transesterification is a polyurethane. Since urethanes can be generated from amines and organic carbonates using the methods of the prior art, the present invention provides processes for the manufacture of polyurethanes from amines which avoid the use of either phosgene or isocyanates both of which are extremely toxic.