Abstract: A system for determining photo-chemical reaction heat evolved from a sample material irradiated with radiant energy comprises a calorimeter having a pair of sample holders connected to heat-leakage type thermo-electric elements which develop electric output signals proportionate to the amount of heat energy conducted therethrough. Both sample holders are initially irradiated with radiant energy before disposing the sample material thereon to heat the sample holders and thereby develop electric output reference signals. The amount of radiant energy being irradiated on at least one of the sample holders is then selectively adjusted to effectively compensate for any difference between the two reference signals.