Time-resolved optical second harmonic generation (SHG) is used to probe the variations in the surface electrostatic fields that occur under pulsed UV (supra-bandgap) illumination at a n-TiO2(001) electrode. The interfacial SHG is dominated-by the electric field-induced second harmonic response from the first 20 nm of the,semiconductor surface; this large SHG signal decreases sharply under UV illumination due to the creation of a steady-state photogenerated hole population at the interface. An additional transient drop in SHG intensity is observed when the pump and probe beams are overlapped temporally on the TiO2 surface. The time scale for this additional band flattening is determined by the finite time required for photogenerated holes to migrate to the interface under the influence of the surface electrostatic fields. An average transit time of 25 ps that is independent of applied potential or solution composition is observed; this corresponds to a hole drift velocity of 4.0 x 10(4) cm s(-1) at the TiO2 surface. The SHG intensity is found to return to its steady-state level in 3-4 ns due to-the removal of the excess holes at the surface by, electrochemical charge transfer and surface recombination processes.