Thin film processes such as etching, deposition, reactive etching, and reactive deposition involve energy transfer which affects substrate temperature measurably. In plasma chemical reactors, these changes are related to ion flux and energy, rate of etching or deposition, and to the specifics of the reactions occurring at the substrate. We have adapted the method of thermal relaxation calorimetry, originally developed for low temperature measurements, to estimate the heats of reaction at the surface of substrates in low pressure plasmas. The technique may also be used to measure deposition and etch rates and for end point detection. Calorimetry can be important in plasma diagnosis and in understanding process mechanisms. This is shown by studies of the variation of the dissociation efficiency of O2in a rf plasma due to the addition of CF4, and of the heat of oxidation of polyimide as a function of gas composition in aO2/CF4microwave plasma downstream etcher. © 1986, American Vacuum Society. All rights reserved.