We investigated the effect of a starch/acetate mixture on the formation of intracellular storage biopolymers compared with system behaviour where these compounds served as a single organic substrate. Three laboratory-scale sequencing batch reactors (SBRs) were operated at steady state with a sludge age of 8 days, one fed with acetate, another with starch and the third with a starch/acetate mixture. The SBR operation involved six cycles per day and continuous feeding during each cycle. Both acetate and starch generated storage biopolymers under continuous feeding. A poly-hydroxybutyrate (PHB) or glycogen pool was formed depending on the selected single substrate. In addition, around 18% of the acetate fed within each cycle was converted to PHB, while the remaining 82% was directly utilized for microbial growth. A higher glycogen formation of 44% was observed for starch. Substrate storage as PHB and glycogen continued with the feeding of the acetate/starch mixture. This observation, supported by microbiological analyses, indicated that the acclimated biomass in the corresponding SBR system sustained microbial fractions capable of performing metabolic functions associated with the formation of the two storage biopolymers. PHB accumulation was reduced as acetate could be more readily used for direct microbial growth in the presence of starch.