The photoinitiated polymerization of dimethacrylate oligomer with 2,2-dimethyl-2-hydroxy acetophenone (Darocur 1173) as radical photoinitiator was studied by using isothermal photocalorimetry. The effect of temperature, light intensity and photoinitiator concentration on reaction was investigated. The maximum conversion was obtained at temperature near 90 degrees C. This temperature is above the glass transition temperature of the resulting material and below the thermal polymerization temperature of the reacting system. Above 90 degrees C, thermal polymerization interferes on photocalorimetric measures. Assuming that glass transition temperature of the final polymer and conversion are connected, we have estimated thermal conversion. A maximum conversion was obtained for a photoinitiator concentration of 1% (w/w) and for the highest light intensity studied (2.7 mW cm(-2)).