In this study diametral tensile strength and microhardness of composites were investigated in relation to various temperatures during curing. It has been demonstrated that temperatures elevated up to 60 degrees C can improve the mechanical properties considerably. Also the stability of the resin system in aggressive environments was improved due to more efficient cross-linking. Thermal analysis of the material showed an endothermic reaction between 60 and 70 degrees C that is believed to be responsible for the initiation of the extra cross-linking. Heating of composite fillings during clinical use is suggested.