Calculations and experiments clarifying the origin of optical losses in photoelectrochemical cells are reported. Losses due to reflection at the interfaces are examined in both front-wall and back-wall illuminated cells, and it is shown that they can be drastically reduced in the latter. Losses due to absorption within non-transparent electrolytes, which exist only in front-wall cells are also considered. The cell photocurrent has been measured as a function of incident light wavelength and, under white light, as a function of window-electrode separation, for three different electrolyte concentrations. Several quantitative conclusions about the cell configuration are drawn.