The effect of aqueous polyselenide solution modification on the photoelectrochemical behavior of n-GaAs/aqueous polyselenide photoelectrochemical cells (PECs) is investigated. Solution modification is achieved through pH control and changing the ratio of dissolved selenium to selenide. Numerical analysis of speciation in various polyselenide solutions is compared to PEC behavior. It is shown that PEC photocurrent, photovoltage, and fill factor are affected by the distribution of hydroselenide, selenide, and polyselenide in solution. There appears to be an optimum pH ([KOH] = 1 to 2M) providing sufficient concentrations of Se-= to improve PEC response. A back wall PEC was designed to investigate the high selenium concentration domain not normally accessible to conventional regenerative solar cells. Photopotential is enhanced by up to 50 mV in high [Se-0] electrolytes. However, these electrolytes can diminish photocurrent stability.