The kinetics of the thermal polymerization of the dicyanate ester Arocy B-10 (4,4’-dicyanato-2,2’-diphenylpropane), following irradiation in the presence of the catalyst tricarbonyl cyclopentadienyl manganese (CpMn(CO)(3)), has been investigated using d.s.c. and FTi.r. It was found that the network developed a higher T-g at lower extents of conversion than observed in uncatalysed thermal polymerization. The rate of the decrease of cyanate concentration was described by a first-order dependence on the active catalyst concentration and the cyanate concentration. The activation energies were calculated for the different irradiated systems, and they decreased with the increase of the irradiation time. Cyanate with a pre-made catalyst system which aimed to simulate the directly irradiated system were also studied, and the kinetic parameters were determined. A mechanism of cyclotrimerization is proposed based on the kinetic data as well as the FTi.r. spectra from a model study. This involves the photo-substitution of the carbonyl groups in the catalyst by the cyanate groups (L) during irradiation to form complexes CpMn(CO)(3-x)L(x) which are suggested to produce the active catalyst by thermal decomposition.