We investigate the relative stabilities of tautomers of 8-oxoguanine (8OG) and guanine (G) comprehensively at various levels of theory up to MP4 and CCSD(T) in the gas phase. We find that the population of the enol form at C6 position in 8OG is higher than that in G at any level of theory which we used. This may partly account for the high mutagenicity of 80G.