The effects of conversion and temperature on the dynamic mechanical properties (at ≈ 1 Hz) of a dicyanate ester/polycyanurate thermosetting system are investigated after cure using torsional braid analysis (TBA). Extent of conversion is measured by Tg. The isothermal glassy‐state modulus at measurement temperatures below the glass transition temperature of the monomer (Tg0) decreases with increasing conversion. The isothermal modulus at temperatures above Tg0 passes through a maximum due to competition between increase in the isothermal glassy‐state modulus at the measurement temperature due to the vitrification process during cooling and the aforementioned decrease in the modulus with increasing conversion, which is considered to be due primarily to steric constraints in the developing network. The maximum in the isothermal modulus is associated with the boundary between the glass and glass transition regions. The experimental results are summarized in a conversion (Tg)–temperature–property diagram, the TgTP diagram, which is a framework for understanding relationships between transitions and material properties for thermosetting systems. © 1994 John Wiley & Sons, Inc.