Polycyclotrimerization of 4,4’-thiodiphenylcyanate was studied by means of differential scanning calorimetry. Samples of different impurity levels or different catalyst loadings (n-nonylphenol (NP), 1 to 10 phr) were studied. In the absence of added catalyst, the sample with higher impurity level (most likely residual water) underwent polycyclotrimerization at a lower temperature range than the sample of comparatively higher purity, although both samples shared the same apparent activation energy (E(a) = 93 kJ mol(-1)) and exhibited similar autocatalysed first-order characteristics. The lowering of the reaction temperature range was more significant in the presence of NP. Interestingly, kinetic features in this case were quite different from the uncatalysed counterpart : in addition to the expected decrease in the apparent activation energy (E(a) = 75 kJ mol(-1), independent of the catalyst concentration), the cure reaction followed an autocatalysed second-order kinetics. These observations are explained in terms of a mechanistic scheme in which the competition between hydroxyl-catalysed and autocatalytic paths is considered. In particular, formation of NP aggregates ((NP)(m), where m approximate to 7) is proposed to account for the less-than-additive catalytic capacity with increasing NP level.