Photo-activated sludge (PAS) systems are an emerging wastewater treatment technology where microalgae provide oxygen to bacteria without the need for external aeration. There is limited knowledge on the optimal conditions for enhanced biological phosphorus removal (EBPR) in systems containing phosphate accumulating organisms (PAOs) and microalgae. The aim of this research was to study the effects of substrate composition and light intensity on the performance of a laboratory-scale EBPR-PAS system. First, a EBPR model was developed to study the effect of organic carbon (COD), inorganic carbon (HCO3) and ammonium-nitrogen (NH4-N) in nitrification deprived conditions on phosphorus (P) removal. Based on the mathematical model, two different synthetic wastewater compositions of COD:HCO3:NH4-N of 10:20:1 and 10:10:4 were examined in laboratory reactors. The performance of the system was also investigated at different light intensities: 87.5, 175, 262.5, and 350 µmol m-2 sec-1. At a ratio of 10:20:1, the system performed poorly, potentially due to an insufficient supply of NH4-N for PAOs growth. At a ratio of 10:10:4, the performance improved significantly as microalgal growth was balanced by reducing inorganic carbon, and was able to operate without external aeration. Under this mode of operation, the net removal of P was 10.33 ±1.45 mg/l and the total P uptake was 27.8 ±1.8 mg/l (94.7±5.7 % P removal). No significant variation was observed in the reactor performance for different light intensities, suggesting that the system is resilient against fluctuations in light intensity.