Chronoamperometric curves under photoexcitation were measured at a polycrystalline TiO2 electrode in iodide-containing solutions using an FTIR spectro-photoelectrochemical cell with a solution layer thickness adjustable from ten, up to about 10 mm with an accuracy of +/-10 mu m. The shape of the curves is controlled by changes in the effective exciting light intensity due to light absorption in solution by generated intermediates and products. The influence of initial electrolyte concentration and solution layer thickness on the shape of the I-t curves is explained.