Polycyclotrimerization of 4,4'-thiodiphenylcyanate was adopted as a model system for general thermosetting polymers for studying the relationship between the glass-transition temperature (T-g) and conversion (alpha) during network formation. Existing expressions for T-g-alpha relationship were used and compared. The experimental T-g-alpha data were well fitted to several one-parameter equations although the physical significance of parametric values thus obtained could not be unambiguously identified. Among the two-parameter models, both the Hale-Macosko-Bair equation and the so-called "original" DiBenedetto equation were well fitted by experimental data (when the mean-field crosslink density was used), yielding parametric values consistent with the original designated physical meanings within the corresponding theoretical frames. Relationships between the parameters in different theories were also discussed. Incidentally, a discontinuity of Delta CpTg at the gel point was observed (i.e., Delta CpTg is of different values in the pregel and postgel regimes, respectively).