Infrared spectra of the mass selected ionic complexes H-2-HCO+ and D-2-DCO+ have been recorded in the vicinity of their nu(1) vibrations (H-2/D-2 stretch) by means of photofragmentation spectroscopy. The anomalous rotational constants obtained by fitting the observed line positions to a semirigid Watson A-type Hamiltonian reflect the appreciable zero-point excursions of the H-2/D-2 molecule. Barriers for this internal motion are estimated utilizing a simple atom-diatom hindered rotor Hamiltonian. According to this one-dimensional model, the barrier increases by about 15% upon vibrational excitation which is mainly attributed to electrostatic effects. (C) 1997 American Institute of Physics. [S0021-9606(97)00344-9].