In vivo¹³C-NMR, ³¹P-NMR techniques were applied to study phosphorus and carbon metabolism in activated sludge during both the anaerobic and the aerobic stages. By supplying a ¹³C label on the methyl group of acetate at the beginning of the anaerobic stage, the fate of the label through the subsequent aerobic/anaerobic stages was traced in vivo. It was possible to follow the flux of label from acetate to hydroxybutyrate/hydroxyvalerate co-polymer in the first anaerobic stage, then to monitor the conversion of these units into glycogen in a subsequent aerobic stage, and afterwards, by submitting the same sludge to a second anaerobic stage, to observe the flux of labelled carbon from glycogen to the hydroxyvalerate and hydroxybutyrate units. The uptake/release of inorganic phosphate and the extracellular pH were monitored by ³¹P-NMR in the same experiments. The data provide an unequivocal demonstration of the involvement of glycogen in the biological phosphorus removal process. On the basis of these ¹³C labelling data, a biochemical model for the synthesis of polyhydroxyalkanoates from acetate and glycogen was elaborated in which the tricarboxylic acid cycle is proposed as an additional source of reduction equivalents. According to this study, from 1 C-mol acetate, 1.48 C-mol P ( HB HV ) are synthesized and 0.70 C-mol glycogen are degraded anaerobically, while 0.16 P-mol phosphate is released. In the aerobic stage, 1 C-mol of P ( HB HV ) is converted to 0.44 C-mol glycogen.