Pyrazoles were found as a novel class of blocking groups for isocyanates. Adhesive mixtures of pyrazole blocked isocyanates and amine terminated prepolymers like Jeffamines(R) (Texaco Chem. Co.) combine excellent reactivities (gelation times within minutes at 100-120-degrees-C), good latencies (more than 170 days at 40-degrees-C), and good adhesion properties on many substrates. The reactivity of pyrazole blocked isocyanates toward nucleophiles increases with the number of electron donor substituents on the pyrazole nucleus and, thus, can be fine tuned by the appropriate substitution pattern. This behavior contrasts sharply with that of phenolic blocked isocyanates, where reactivities with the same nucleophiles decrease with more and stronger electron donor substituents on the phenol nucleus. Therefore, different deblocking reaction mechanisms were proposed for pyrazole vs. phenol blocked isocyanates. The excellent latency of pyrazole blocked isocyanate/Jeffamine mixtures is due to the insolubility of the two components at ambient temperature and the slow endothermic dissolution process at higher temperature. The good adhesion of formulations with pyrazole blocked isocyanates as reactive components on most plastic and metal substrates is ascribed to the primer action of the released blocking group.