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A xenon time projection chamber with an active volume of 207 L has been built to study neutrinoless double p decay in '^^Xe. Data were taken in the Gotthard Underground Laboratory, with 5 atm of xenon enriched to 62.5% in '^^Xe. From 3380 h of data, no evidence has been found for the Ov O"*"-* 0"^ transition. Half-life limits of r?/2 > 2.5(4.9) x 10^^ yr in the mass-mechanism mode and T'^/2 > 1.7(3.2) X 10^^ yr in the right-handed-current mode, at the 90(68)% C.L., were derived. An upper limit for the Majorana neutrino mass parameter was deduced.