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Measurements have been made on grown germanium p-n junction diodes in an attempt to correlate the excess reverse current that occurs under exposure to water vapor with the presence of an n-type surface-conductivity layer, or "channel," on the p-side of the junction. Using an optical method to measure the length of this surface layer, it has been found that (1) no channel is formed on a freshly CP4-etched surface, and that (2) for well-oxidized surfaces, large excess reverse currents occur which are directly proportional to the length of the channel for all measured values of applied voltage and humidity. By combining this observation with the known behavior of the channel conductivity with humidity and voltage, a formula for the excess current as a function of the applied bias is derived. In spite of several oversimplifications in the model, the predicted values agree reasonably well with the experimental results for biases greater than one volt.