The infrared spectrum of H3O+ in positive column discharges of H2/O2 gas mixtures has been studied by a difference frequency laser spectrometer. The nu1 fundamental band of H3O+ was identified in the region of the strong. Molecular constants were obtained by the least-squares fitting of the observed frequencies, and band origins of the and subbands were determined to be 3389.656(2) and 3491.170(2) cm-1, respectively. During this study, assignment of the nu3 fundamental band was extended to higher J, K transitions, which do not fit to the calculated pattern well, but have definitely been assigned by using the ground state combination differences and relative intensities. Vibration-rotation interactions between the nu1 and nu3 states have been considered, which explained some large discrepancies between observed and calculated frequencies and led to the identification of forbidden transitions. Energy differences in the ground state between the DeltaK = 3 rotational levels were obtained from the combination differences of the forbidden and allowed transitions, which led to an accurate determination of C and DK. Equilibrium structure of H3O+ has been derived to be re = 0.974(1) Å and alphae = 113.6(1) degrees. Copyright 1999 Academic Press.