Exposing active Spindt-type molybdenum field emission microcathode arrays to CH4 resulted in increased electron emission. Exposures of 250 and 2500 L result in electron emission enhancement of 106.2 +/- 41.0% and 200.1 +/- 20.7% and a decrease in tip work function by 10.2 +/- 5.6% and 17.6 +/-1.7%, respectively. The measured work function changes indicate the formation of molybdenum carbides on the field emitter tips.