In order to investigate the fundamentals of electron migration in nanostructured metal-oxide semiconductors, the transient photocurrent response of dye-sensitized porous nanocrystalline TiO2 is studied. The time-resolved photocurrent response at light steps or pulses shows a faster transient upon increasing the light intensity. Intensity-modulated photocurrent spectroscopy (IMPS) reveals that the transient photocurrent is dominated by two time constants, i.e. the geometrical one and a characteristic time related to electron trapping. A theoretical model is derived in which the occupation dynamics of a single electron trap is considered using Shockley-Read-Hall kinetics. The geometrical RC time of the electrode is also included. Excellent agreement between this model and the measured IMPS spectra is obtained.