This work describes the generation of volatile fatty acids (VFAs) as the first step of the polyhydroxyalkanoate (PHA) production cycle. Therefore four different substrates from a municipal waste water treatment plant (WWTP) were investigated regarding high VFA production and stable VFA composition. Due to its highest VFA yield primary sludge was used as substrate to test a series of operating conditions (temperature, pH, retention time (RT) and withdrawal (WD)) in order to find suitable conditions for a stable VFA production. The results demonstrated that although the substrate primary sludge differs in its consistence a stable composition of VFA could be achieved. Experiments with a semi-continuous reactor operation showed that a short RT of 4 d and a small WD of 25% at pH = 6 and around 30 degrees C is preferable for high VFA mass flow (MF = 1913 mg(VFA)/(L d)) and a stable VFA composition. (C) 2013 Elsevier Ltd. All rights reserved.