Аннотация:The free energies are calculated for some interactions of gallium arsenide with cesium. It is found that photocathodes with a negative electron affinity, made from gallium arsenide with cesium adsorbed on the surface, are not thermodynamically stable systems under synthesis and use conditions. Starting from the free energy values, the maximum cesium vapor pressures above the photocathode, at 298 degree K are estimated for which no interaction of the cesium with the gallium arsenide will occur. The increase in photocathode stability as its surface oxidizes is explained qualitatively.