A novel electrode geometry and contacting procedure has allowed measurement of the quasi-Fermi levels, i.e., the apparent electrochemical potentials, of electrons and holes at an illuminated semiconductor/liquid contact. The key feature of our experiments is the use of a lithographically patterned, high purity (100-400 Ohm.cm n-type float zone material), low dopant density Si sample in contact with CH3OH-dimethylferrocenec(+/0) solutions. The photogenerated carriers can be collected at the back side of the Si sample through a series of diffused n(+) and p(+) points. The lifetime of photogenerated carriers approaches 2 ms in this sample, indicating that electron-hole recombination is minimized in the bulk of the semiconductor. Furthermore, surface recombination is minimized by use of low saturation current density, ohmic-selective contacts at the back of the sample. The solid/liquid contact also has a low recombination rate. Therefore, the potentials measured at the diffused points yield values for the quasi-Fermi levels of electrons and holes under illumination of the semiconductor/liquid contact. Transient photovoltage measurements have also been performed to confirm quantitatively that the quasi-Fermi levels are flat across the Si samples used in this work.