Recent advances in high resolution X-ray photoemission spectroscopy (HRXPS) have enabled one to directly probe an electron transfer reaction between adsorbates and model catalyst surfaces. For example, on cesium adsorbed molybdenum disulfide surface it has been observed that deposition of submonolayer cesium onto the basal plane of clean MoS2 introduces a new electron density of states at 1.25 eV above the MoS2 valence band maximum (VBM). Combined density functional theory and angle-resolved HRXPS indicate that the new VB state originates mainly from the bottom of the MoS2 conduction band, mixed to a small extent with the Cs 6s orbital. It has been further demonstrated that the supra-valence electrons in MoS2 can be captured and reacted with electron acceptor molecules forming surface ionic species. On the other hand, the interface between Li and MoS2 exhibits a quite different behavior - a reductive reconstruction. In this talk, I will review recent findings from alkali metal adsorbed, layered transition metal sulfide surfaces and discuss the on-going effort to understand the underlying principle for the adsorbate-substrate interaction.