The HOCO radical has been successfully detected by a photoionization mass spectrometry in gas phase. The HOCO radical produced by the reaction of Cl+HCOOH (with small excess energy; 13.0 kcal mol-1) was found to be stable and its lifetime was confirmed to be very long (> 10 ms) at room temperature. On the other hand, the HOCO radical formed by the 193 nm photolysis of C 2H3COOH (with large excess energy; 55.8 kcal mol -1) exhibited fast, nonexponential, and total pressure-dependent decay. This behavior was well interpreted by a competition of dissociation and relaxation of "hot" radicals. Further, the decay of "hot" radicals showed an apparent isotope effect (DOCO/HOCO). This fact together with an energetic consideration suggests that the observed decay is dominated by a tunneling dissociation to H+CO2, which implies the importance of the tunneling in the OH+CO reaction. The vibrational relaxation rate of the HOCO radical was roughly evaluated from the pressure dependence of the decay profile of "hot" HOCO radicals. The rate of reaction of HOCO radical with molecular oxygen has also been determined, and appeared pressure-independent.