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In this study, the optimization of polyhydroxyalkanoates (PHA) production using acidified dairy wastewater (DW) and organic food waste (OFW) as a substrate source was investigated. The study included (a) acclimation of activated sludge in two sequencing batch reactors operated under different operational conditions (feast-famine and anaerobic-aerobic-anoxic) and fed with domestic wastewater only and with volatile fatty acid (VFA) supplement, (b) acidification experiments designed for the initial substrate to inoculum (S/I) ratios of 1, 2.5, 5 and 10 g tCOD/g VSS (tCOD - total chemical oxygen demand and VSS - volatile suspended solids) using DW and OFW, (c) batch experiments for PHA production using each acidified substrate source. The maximum acidification yields were obtained at the S/I ratio of 10 as 51.7% and 46.6% for DW and OFW at 4th d. PHA contents of up to 67.3% and 4.5% of dry cell weight were obtained using VFA obtained from acidification of DW and OFW, respectively. Accordingly, the results showed that acidification of 1000 m3 of DW and one tone of the wet weight of OFW could produce 173 and 20.3 kg of polymer, respectively. This could be attributed to a significant profit when a large amount of DW and OFW is considered. [ABSTRACT FROM AUTHOR]