AbstractThe preparation of blocked isocyanates and their thermal dissociation into isocyanate and blocking agent was studied by DSC and IR spectroscopy. Thermal deblocking was found to be possible only with isocyanates blocked with ε‐caprolactame or 2‐butanone oxime, but not with β‐ketoesters. With 2‐butanone oxime — the most convenient blocking agent — the dissociation of blocked aromatic isocyanates starts at ≈ 80°C and is fast at 120°C. With blocked aliphatic isocyanates, the temperature of dissociation (Td) is higher (≈ 120°C). Ether or tertiary amines have only a poor catalytic effect on the dissociation, and metal naphthenates have no apparent effect. The thermal and viscosity properties of prepolymers with terminal blocked isocyanate groups were investigated. The molecular interactions decrease in the order 1,4‐phenylene diisocyanate (7) ≫ 4,4′‐methylenediphenylene diisocyanate>trans‐1,4‐cyclohexylene diisocyanate ≈