In this study, the enzymatic oxidation of acetaldehyde, one of the substrates of xanthine oxidase, has been studeid by a microcalorimetric method. A flow microcalorimeter is used to measure the heat changes during the during the reaction. The catalytic determination of xanthine oxidase between the concentrations 0.2×10−1and 2.4×10−1mg/ml (0.8×10−3−9.6×10−3U/ml) has been achived keeping acetaldehyde concentration constant at 4.0×10−3M and the catalymetric determination of acetaldehyde between 0.8×10−3and 2.8×10−3M is accomplished at a fixed concentration of enzyme.