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n+-GaP/stearic-acid/Au MIS structures have been fabricated by depositing monolayers on the chemically etched semiconductor surface using the Langmuir-Blodgett technique. Their electronic and photoelectronic properties are compared with those of n+-GaP/Au diodes. A single monolayer of stearic acid increases the open-circuit photovoltage developed by such devices from 400mV to 900mV without significantly altering the short-circuit photocurrent. The dark I/V characteristics under forward bias conditions and the photoelectric action spectra both show near-ideal Schottky-barrier diode properties and the barrier height as measured by both methods increases from 1.14eV without the monolayer to 1.58eV in its presence. A number of other organic monolayers give very similar results. In the presence of multilayers of stearic acid the behaviour of the MIS device becomes complex and deviates considerably from ideality. In general, the photocurrent due to over-the-barrier excitation is much more affected by the nature and thickness of the interfecial layer than the band-to-band photocurrent. A plausible explanation is offered for the increased barrier height in the presence of a monolayer, based on establised Schottky-barrier theory, but the full range of behaviour observed, and particularly that with multilayers, cannot yet be adequately treated theoretically.