A new procedure has been defined for the respirometric assessment of bacterial storage yield as defined in the Activated Sludge Model No.3. The procedure was used to determine the storage yield, Y(STO), associated with acetate, glucose and domestic sewage, together with mixtures of acetate/glucose and acetate/domestic sewage at different initial F/M ratios. Y(STO) was calculated as 0.78 gCOD(gCOD)(-1) for acetate, 0.87 gCOD(gCOD)(-1) for glucose and 0.96 gCOD(gCOD)(-1) for domestic sewage. The Y(STO) of substrate mixtures was found to reflect the characteristics of the dominant fraction in the mixture.