rigid spiro aromatic dicyanate, 6,6＇-dicyanato-3,3,3 ＇,3＇-tetramethyl-1, 1＇-spirobi-indane (DCSI), was synthesized and its polycyclotrimerization investigated. The critical gel point (alpha(gel)) during polycyclotrimerization of DCSI was studied by gravimetric analysis. The resulting value of alpha(gel) ( = 65 +/- 2%) deviates greatly from the theoretical value of 50%. To explore the possible reason for this high deviation, partially cured samples were analysed by fast atom bombardment mass spectrometry (FAB-MS). The predominant formation of trimer at a conversion ＇ 50% is responsible for the delayed gelation. In addition to the small amounts of side products such as dimer and carbonate imine, the formation of products with internal cycles (i.e. tetramer and hexamer) is another reason for the high alpha(gel).