We performed a comprehensive study of Cu(In,Ga)Se2 (CIGS) surface reactivity in air, focusing on the evolution of the surface chemistry via X-ray photoemission spectroscopy (XPS). By using the different transitions (photopeaks and X-ray Auger electron spectroscopy transitions) available on XPS spectra, in order to probe different surface thicknesses, complementary chemical information is provided to investigate the surface reactivity mechanism of CIGS surface and especially highlight the importance of the relative humidity rate of the atmosphere. Indeed, by maintaining a relative humidity of 20%, we demonstrate that 2 this degradation can be limited, even almost stopped, showing that the degradation mechanism is not only related to O2 interaction but more importantly, to the thin water layer, inherent to relative humidity, formed upon surfaces.