Electron emission from molybdenum field emission microcathode arrays is sensitive to the contamination of the emitters by the residual gases. We studied the effects of H2O exposures on the emission properties of several Spindt-type molybdenum field emitter arrays. Exposure of the field emitters to H2O resulted in emission degradation ranging from (13.8+/-5.7)% for 50 L to 90.7% for a 4000 L exposure.