K-Te photocathodes deposited on a Mo substrate have been successfully used as an electron source in the free electron laser of University of Twente. Long lifetimes have been measured: after more than 20 h of operation in the accelerator a K-Te cathode with 4.75% initial quantum efficiency still displays a 1.1% quantum efficiency at 259 mm. Moreover, the quantum efficiency of this cathode versus operation time can be fitted by an exponential decay curve, which saturates asymptotically to a 1.03% value, suggesting that a quantum efficiency close to 1% could be sustained for very long operation times. Films degraded by use can be recovered to a quantum efficiency which is close To the initial value, by heating the substrate al temperatures between 100 and 330 degrees C. A new procedure to obtain K-Te cathodes with high (up to 11%) quantum efficiencies is described. (C) 1997 American Institute of Physics.