The spin polarization of photoelectrons emitted from optically pumped negative electron affinity GaAs shows a systematic depolarization. This is attributed to the spin‐flip scattering at the Cs‐O‐Cs overlayer. The photoexcited electrons thermalize to the &Ggr; point prior to their escape and hence go through the overlayer with the same kinetic energy resulting in photon‐energy‐independent polarization loss.