The resultant exotherms obtained from the photopolymerization of thin‐film chemical samples have been investigated. The radiant source used is a standard 200‐W in.−1mercury discharge lamp. About 55% of this power is in the infrared (IR) part of the spectrum, which is removed by a water‐cooled quartz tube. Different regions of the remaining spectrum were selected in the range 270–450 nm with cuton filters. A grating spectrometer is used to measure the transmitted spectra. A thin foil heat flow sensor is used as a calorimeter to measure the heat given off by a chemical sample when exposed to 0.5‐s pulses of ultraviolet (UV) radiation. The pulse is formed by means of a bistable electronic shutter. Data were taken for samples ranging from 0.0125 to 0.05 mm [0.5 to 2.0 milli‐inch (mils)] thicknesses. The heat output per gram is largest for the thinner samples and the absorption characteristics were observed to depend on the thickness of the sample.