With acetate or propionate as the sole carbon source, anaerobic metabolic models describing phosphorus- and glycogen-accumulating organisms (PAO and GAO) have been developed in the literature. However, comprehensive models are in need for the description of PAO and GAO behaviors with mixed acetic and propionic acids as carbon sources since they are the two main volatile fatty acids (VFA) that coexist in real wastewater. Two metabolic models were proposed to characterize the anaerobic stoichiometry of PAO and GAO, respectively, and two groups of sequencing batch reactors (i.e. 5 PAO-SBRs and 5 GAO-SBRs) with different propionic to acetic acid ratios were used for the validation of the models. The experimental data indicated that polyhydroxyalkanoates were synthesized via random condensation in GAO cells, whereas the semi-selective/semi-random pathway was used for the integration of acetyl-CoA and propionyl-CoA in PAO cells. When the VFA was pure acetic or propionic acid, the proposed PAO (or GAO) model reverted back to the reported acetate or propionate PAO (or GAO) model. Results also showed that the energy required for the transportation of 1C-mol VFA across the membrane of both PAO and GAO cells was independent of the propionate/acetate ratio.