Measurements have been performed on the degradation of emission from silicon field emitting devices in ultrahigh vacuum (UHV) and vacuums containing CO2 and CH4 with anode potentials of 0 and 2500 V. Degradation is very fast in CO2, slower in CH4, and very slow in UHV. Analysis of the data using a numerical simulation shows that, in UHV, the degradation can be explained by an increase in work function with time. For CO2 and CH4, however, it is primarily due to a blunting of the tips that is partly compensated for by a decrease in work function as emission progresses. There is some evidence for tip sputtering being important in the case of CO2 at high anode voltages.