In an effort to gain further insight into the oxidation of the phenyl radical, this contribution details the first of three experiments designed to establish the heat of formation of the 2-oxepinoxy radical. We report here the synthesis of the previously unknown 2(7H)-oxepinone (12a) and 2(3H)-oxepinone (12b). We have determined the gas-phase acidity (Delta(acid)H(298)) of 12b by means of a bracketing study employing a flowing afterglow apparatus with quadrupole mass spectrometric detection. In this experiment, compound 12b was reacted in the gas phase with a series of bases of varying strength. A proton-transfer reaction was observed when 12b was reacted with t-BuS-, but not when 12b was reacted with HS-. We conclude that the gas-phase acidity of 12b lies between those of t-BuSH and H2S, and it is thereby assigned a value of Delta(acid)H(298) = 352 +/- 2 kcal/mol. Additional support for this value was found by performing the reverse reactions (i.e. the 2-oxepinoxy anion (15a) was reacted with proton sources of differing acidities). Anion 15a underwent a proton-transfer reaction with H2S but not with t-BuSH, in agreement with the results from the forward reactions. The experimental value of the gas-phase acidity agrees well with those from DFT calculations, which predicted Delta(acid)H(298) = 348.9 kcal/mol at the B3LYP/6-31 +G(d) level and 349.2 kcal/mol at the B3LYP/aug-cc-pVTZ level.