A microwave digestion procedure has been evaluated for use in the determination of chemical oxygen demand (COD) by the dichromate method, using small volumes of sample and reagents. Different organic substances with COD values between 50 and 500 mg O2.1(-1) were analyzed by both the proposed procedure and the standard method, obtaining satisfactory results (mean recoveries 91-102%) with digestion times lower than 5 min. Samples of river water and wastewater were also analyzed, obtaining results showing good agreement with the standard method.