The present IR-UV depletion spectroscopic study extends the recent data of literature by providing evidence for the existence of a fourth form of guanine in the gas phase. The comparison of the UV and IR signatures of the four forms together with those of the 7- and 9-inethylated derivatives allows us to build up a new assignment in terms of enol/keto and 7/9NH tautomerisms. From this complete picture, it turns out that the UV spectroscopy of free guanine is mainly controlled by the 7/9NH tautomerism: both 7NH tautomers observed are red-shifted compared to the 9NH ones, with the following origin transition order, from red to blue: 7NH enol (32864 +/- 5 cm(-1)), 7NH keto (+405 cm(-1)), 9NH keto (+1046 cm(-1)), and 9NH enol (+1891 cm(-1)); 7-, 9- or 1-methylations are found to cause only moderate red shifts (less than 400 cm(-1)). The opposite trend is observed for the IR spectroscopy, which appears to be essentially controlled by the enol/keto tautomerism. This study exemplifies the need for cross-checked experimental approaches, namely the IR[UV depletion spectroscopy or the study of relevant methylated species, to reach a global and consistent assignment, even in rather simple biological systems such as purine bases.