The effect of the initial molar ratio of the reactive groups, r(NCO) (= [NCO]0/[OH]0 = 1.0 or 0.8), on the gelation of polyurethane networks based on tris(4-isocyanatophenyl)thiophosphate and poly(oxypropylene) diol, was studied. The number-average molecular weight, M(n)BAR, in the pre-gel state, the critical conversion at the gel point, p(NCO)c, and the critical ratio, r(NCO)c, at which gelation occurs, were measured. The decrease in 1/r(NCO)c with increasing dilution in xylene suggests that cyclization takes place. The experimental M(n)BAR values are always lower than the theoretical ones, with the latter being calculated for the ring-free case. The fraction of bonds that are lost in the cyclization process, s, calculated from the difference between the experimental and theoretical values of M(n)BAR increases with conversion and has a value near the gel point of 0.04-006. It is demonstrated both theoretically and experimentally that the values of s calculated from the shift of the gel point conversion are not equivalent to those obtained from M(n)BAR, but depend on r(NCO).