AbstractIn 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 0.2×10⁻¹ and 2.4×10⁻¹ mg/ml (0.8×10⁻³ −9.6×10⁻³ U/ml) has been achived keeping acetaldehyde concentration constant at 4.0×10⁻³ M and the catalymetric determination of acetaldehyde between 0.8×10⁻³ and 2.8×10⁻³ M is accomplished at a fixed concentration of enzyme.