Thin GaS films deposited on B-doped diamond (as-grown) and on oxygen-annealed B-doped diamond (O-ann.) films grown by plasma-assisted chemical vapor deposition (CVD) on p(+)-type Si(001) substrate were characterized by Auger electron spectroscopy (AES), and X-ray photoemission spectroscopy (XPS). The thermal evaporation of GaS single crystal is used for the deposition. Ga and S Auger peaks are observed in the AES spectra of both GaS-deposited samples. and only weak S-Auger peaks are observed in the samples post-annealed at about 900 degrees C. XPS spectra indicate an upward band bending due to GaS deposition and the increase in the downward band bending due to post-annealing at higher temperature. C-S and C-Ga bonds an observed for GaS-deposited diamond films. Secondary electron emission (SEE) spectra indicate that the S-terminated diamond films seem to possess positive electron affinity (PEA) surface.