A generalized theoretical model of intensity modulated photocurrent spectroscopy (IMPS) is proposed, and analytical equations for the frequency dependence of the photocurrent and photovoltage responses are derived. The theory treats the influence of the space charge, the surface state and the Helmholtz capacitances on the IMPS response. In the model, charge transfer and recombination at the semiconductor/electrolyte interface are described by kinetic equations, whereas surface state charging and the current through the external circuit are treated using linear circuit equations. The calculated IMPS plots exhibit a number of useful diagnostic features. The treatment also shows that it should be possible to measure the space charge capacitance directly from the photovoltage response, even if it is comparable in magnitude with the Helmholtz capacitance. Equivalent circuits representing the processes occurring during IMPS measurements are discussed, and it is shown that single passive elements do not provide a satisfactory description of the kinetic steps responsible for the photocurrent response.