The difference in heat capacity of the liquid state and that of the glassy state at the glass transition temperature, Delta C-p(T-g)=C-p(l) - C-p(g), has been determined in samples of epoxy resin with different extents of cure. The epoxy was a diglycidyl ether of bisphenol A cured isothermally by phthalic anhydride as hardener in a temperature range between 30 and 130 degrees C for various periods of time. As crosslinking density increases, the Delta C-p(T-g) decreases. This result has been interpreted within the framework of ’strong’ and ’fragile’ behaviour of glass-forming systems. The decrease of Delta C-p(T-g) with increased crosslinking suggests a resistance of the glassy structure to the breakdown due to a ’strengthening’ effect of the crosslinked structure.