A simple, quantitative relationship between the semiconductor surface recombination velocity (S(r)) and the open-circuit photovoltage (V(oc)) of photoelectrochemical systems was derived and verified experimentally. Experimental results obtained for the n-Si/acetone-FeCp2+/0-LiClO4 junction indicate that V(oc) is controlled by surface recombination. Quantitative analysis of the results using the derived expression yields values of the barrier height and S(r) that compare favorably with published data. Application of the equation to other semiconductor/liquid junction cells suggests that the expression may be important in evaluating and understanding the behavior of a wide range of photoelectrochemical systems.