The mean transverse energy (MTE) of electrons emitted from GaAs photocathodes was measured systematically using a new method based on adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field. Electrons with energies above the conduction band minimum are found to be thermalized with the lattice temperature of the cathode while electrons having suffered energy losses prior to their emission show enhanced transverse energies. For (Cs,O) and (Cs,F) activation layers on the same cathode the same MTE is found. By cooling the cathode with liquid nitrogen, the MTE of the high-energy electrons was reduced to≈14 meV. ©1997 American Institute of Physics.