Photocathodes activated to negative electron affinity with a cesium-based activation layer, such as GaAs and GaN, can be used for generating spin-polarized electron beams, but their extreme sensitivity to chemical poisoning limits their operational lifetimes. This work demonstrates that applying and subsequently heating a cesium iodide (CsI) coating can produce a more durable activation layer lacking iodine, but rich in stable cesium suboxides (formal O oxidation state > − 2 ), which significantly extend the dark lifetimes of both GaAs and GaN photocathodes. Through x-ray photoelectron spectroscopy, we examine the stability and formation of these Cs suboxides, which exhibit remarkable resistance to chemical poisoning. Additionally, we investigate the subsequent surface quality using atomic force microscopy. Our findings show that CsI-based surface treatments not only prolong photocathode lifetime but also maintain high spin polarization, positioning this method as a promising approach for enhancing photocathode durability in demanding applications.