Several rubber (acrylonitrile-butadiene copolymer) or thermoplastic (polyethersulfone) additives bearing different chain ends were introduced into pure aromatic dicyanates. We studied the influence of these initially misciblemodifiers on the polymerization kinetics, as a function of their chemical structure and concentration. It appears that apart from those bearing a labile hydrogen atom, the additives play almost no role on the polycyclotrimerization rate;neither does phase separation. However, the additives influence the structure of the final networks insofar as they partially dissolve in the matrix and thus modify both the final T, and the onset of vitrification, compared with the pure monomer. Finally, the amino- or phenolic additives can react with cyanate functions and produce adducts which are well-known catalysts for cyanate cyclotrimerization; in their presence, a marked enhancement of the polymerization rate can thus be observed, together with the thermal effects mentioned above.