During thermal decomposition of methylene-4,4'-di(ethylphenylcarbamate) (MDU), methylene-4,4'-di(phenylisocyanate) (MDI) appears to be the main product, provided that the ethanol formed in the reaction is carried away. Polycarbodiimides are the main by-products along with small amounts of 4,4'-di(aminophenyl)methane and its derivatives. Under the reaction conditions used in these studies, the latter compound together with its derivatives, reacts both with MDI and MDU. At 220-310 degrees C and a reaction time of 80-110 min, the selectivity of the MDU transformation to MDI in relation to consumed MDU is 60-87 mol.%. The selectivity of the transformation to polycarbodiimides is 7-20 mol.%, and that to amines and urea derivatives is about 1-2 mol.%.