A methodology is reported for monitoring the evolution of isothermal physical properties of thermosetting systems with increasing conversion (cure). For this report, measurements of modulus vs. temperature of a single composite specimen were obtained on repetitive cooling and subsequent heating to successively higher temperatures : Cooling and subsequent heating data often displayed thermohysteresis. Cross-plotting provided plots of isothermal modulus vs. conversion (as measured in the same experiment by T-g) for a wide range of temperatures (i.e., from -180 to +380 degrees C) from both the cooling and the heating data. Comparison of isothermal cooling and heating data provided a measure of toughness of the matrix vs. conversion through the extent of hysteresis of the data. The basis of the approach depended on five factors : (a) use of a composite specimen, (b) the ability to obtain data both on cooling and on heating, (c) the effect of microcracking (which is presumably responsible for the observed hysteresis) could be removed by heating ("healing") near the glass transition temperature, (d) the use of a single specimen provided an internal reference for comparing data, and (e) the assumption that T-g is a direct measure of conversion. The methodology is demonstrated using a complex thermosetting isocyanate/epoxy/glass fiber composite system using the TEA torsion pendulum technique.