The solvent-effect together with the influence of substituents in the aryl group on the yield of isothiocyanates in the decomposition of N-mono aryl thioureas can partly be explained by solvent-and substituent-effects on a consecutive reaction of two decomposition products amine and isothiocyanate. A more detailed investigation of this step, which is representative for the reaction scheme of thioureas, shows that reactions of aromatic amines with aryl isothiocyanates are characterized by an equilibrium limiting the yield of the product NN' diaryl thiourea. Both amine and product seem to effect about equal catalytic influences on both steps of the equilibrium. From a study of the solvent-effect on the reaction rate, it can be concluded that besides the unspecific factors, polarity and polarisability, also association-effects and catalytic activities of the solvent must be taken into account.The considerable influence of the basicity of the amine on the reaction rate together with the pronounced effect of substituents in the phenyl isothiocyanate shows that the nucleophilic attack of the amine on the isothiocyanate must be the rate determining step; this view is in agreement with the low value for the isotope-effect. The whole of the data can be correlated by assuming a reaction scheme wherein amine attacks the isothiocyanate both as a dimeric form and a monomeric form complexed with reaction product or solvent.