A respirometric method for determination of the readily biodegradable chemical oxygen demand (COD) concentration in a wastewater was developed, and used to study the effect of the anaerobic stage of a biological phosphorus (P) removal process. The sludge used was obtained from a laboratory scale activated sludge A/O (anaerobic/aerobic) process for biological P removal, fed with synthetic wastewater composed by glucose and peptone as carbon sources. Respirometric tests were made in an electrolytic respirometer. Wastewater short‐time biochemical oxygen demands were measured after anaerobic reactions using the sludge from the A/O process, and the readily biodegradable COD concentrations were then calculated. Several tests were made using different kinds of synthetic wastewater in order to know the accuracy of the experimental technique. These wastewaters were prepared using a constant COD level but different organic composition in order to change their biodegradability. The method was used to study the effect of the anaerobic period duration on the readily biodegradable COD concentration produced from a starch containing wastewater. A higher anaerobic retention time caused an increase in the readily biodegradable COD concentration. This determination can be useful to know the effectiveness of the anaerobic period and thus, the viability of the Biological Phosphorus Removal (BPR) process with slowly biodegradable wastewater.