The long-term stability and the effect of humidity on a solid polymer electrolyte (SPE) electrochemical oxygen sensor have been investigated. The sensor exhibits good stability and fast response in room air at ambient temperature over a reasonable humidity range during a five-month period of time. When the relative humidity is either very high, i.e., higher than 96%, or very low, i.e., lower than 32%, the sensor performance deteriorates. This deterioration is, however, temporary, and the sensor performance can be restored by placing the sensor in a reasonable humidity environment. The water-retention capacity of the solid polymer electrolyte is affected by the humidity in the environment, contributing to the instability of the sensor.