The study investigated the effect of feeding regime and sludge age on acetate utilization. Parallel sequencing batch reactors (SBRs) were operated at steady-state with pulse and continuous feeding of acetate at sludge ages of 8 and 2 days. Acetate was always partially converted to poly-beta-hydroxybutyrate (PHB). The adopted model remained equally applicable to oxygen uptake rate and PHB profiles reflecting different feeding regimes and culture history. Sludge age was significant on the rate parameters of storage and direct growth (k(STO), mu(H1)). while the feeding regime affected half saturation coefficients (K-STO, K-S1). Changing the sludge age from 8 days to 2 days reduced the K-STO value from 8.0 day(-1) to 6.5 day(-1) and increased the corresponding mu(H1) value from 1.5 day(-1) to 2.5 day(-1), regardless of the feeding regime; conversely, changing from pulse to continuous feeding reduced K-STO while increasing K-S for the SBR operation at the same sludge age. (C) 2011 Elsevier Ltd. All rights reserved.