Abstract: We have performed AES, XPS and UPS measurements of Type S and Type M tungsten dispenser cathodes as well as a model surface consisting of a barium monolayer on a tungsten strip in various stages of oxidation. The results indicate the presence of considerable difference between the Type S and Type M cathodes. It was found that Type M cathode is Ba-O-W system, while Type S cathode seems to be partial barium and partial barium oxide and oxygen coadsobed on tungsten. The oxygen may induce change of chemical state of surface barium and substrate tungsten, and change work function and emissive ability of cathodes. The barium layer in Type M cathode appears to be more insensitive to oxidation as indicated by core level changes tham that in the Type S cathode. The valence band structure is also less sensitive to oxygen than Type S cathode. The tungsten related features are more sensitive to oxygen compared to osmium.