<ns3:p><ns3:bold>Background:</ns3:bold><ns3:bold> </ns3:bold>The effect of solid retention time (SRT) over cheese whey substrates in a fermentation process drives changes in the composition of polyhydroxyalkanoates (PHAs) obtained. Volatile fatty acids produced in the first step of an anaerobic sequencing batch reactor were used as substrates to produce PHA using mixed microbial cultures under aerobic dynamic feeding conditions.</ns3:p><ns3:p> <ns3:bold>Methods: </ns3:bold>Analytical methods were used for the standard analysis of parameters of interest including measuring the amount of ammonium and phosphate, chemical oxygen demand, among others.</ns3:p><ns3:p> <ns3:bold>Results</ns3:bold>: The SRT increasing from 4 to 6 and 10 days produced changes in the distribution of volatile fatty acids produced. The polyhydroxybutyrate-hydroxyvalerate copolymers formed in the accumulation stage gave the following results: 58:42, 68:32 and 81:19 (%), referred to SRTs of 10, 6 and 4 days, respectively. The maximum PHA accumulation obtained at 10 days of SRT was 52% of the dry cell weight within 7 h, reaching a PHA productivity of 0.62 g L<ns3:sup>−1</ns3:sup> h<ns3:sup>−1</ns3:sup> and a storage yield of 0.37.</ns3:p><ns3:p> <ns3:bold>Conclusion:</ns3:bold><ns3:bold> </ns3:bold>The SRT variation impact on the distribution of volatile fatty acids in the acidogenic reactor and consequently on the PHA production and composition formed in the accumulation stage.</ns3:p>