In this study, various surface cleaning techniques for the removal of carbon (C) and oxygen (O) from AIN and GaN were investigated. Auger electron spectroscopy (AES) and secondary mass ion spectroscopy were used to monitor the presence of surface C and O, and atomic force microscopy was used to monitor surface roughness. AES analysis showed that ex situ ultraviolet/ozone (UV/O-3) and wet chemical treatments based on HF and HCl were very effective in removing surface C and reducing the native oxide on both AIN and GaN, After H-2 and N-2 plasma treatments in ultrahigh vacuum at temperatures of 750 and 900 degrees C, clean GaN surfaces could br achieved with in the detection limits of AES. An oxygen-free AW surface could not be obtained within the detection limits of AES. SIMS analysis showed that concentrations of surface C and O up to 3 x 10(20) and 2 x 10(22) cm(-3), respectively, still exists on plasma-treated GaN. The results of this study indicate that ex situ UV/O-3 followed by H-2/N-2 plasma treatment is highly effective in reducing the C and O contamination at the GaN surface, bur that further in situ methods are needed to obtain clean GaN and AlN surfaces. None of the various cleaning methods were found to affect the surface roughness.