A combined experimental and theoretical study of the structural properties of the H2–HCO+ ion‐neutral complex has been undertaken. Infrared vibrational predissociation spectra of mass selected H2–HCO+ complexes in the 2500–4200 cm−1 range display several vibrational bands, the most intense arising from excitation of the C–H and H2 stretch vibrations. The latter exhibits resolved rotational structure, being composed of Σ–Σ and Π–Π subbands as expected for a parallel transition of complex with a T‐shaped minimum energy geometry. The determined ground state molecular constants are in good agreement with ones obtained by ab initio calculations conducted at the QCISD(T)/6–311G(2df,2pd) level. The complex is composed of largely undistorted H2 and HCO+ subunits, has a T‐shaped minimum energy geometry with an H2...HCO+ intermolecular bondlength of approximately 1.75 Å. Broadening of the higher J lines in the P and R branches of the Π–Π subband is proposed to be due to asymmetry type doubling, the magnitude of which is consistent with the calculated barrier to H2 internal rotation. The lower J lines in the Σ–Σ and Π–Π subbands have widths of 0.06 cm−1, around three times larger than the laser bandwidth, corresponding to a decay time of ≊90 ps for the upper level. The absence of discernible rotational structure in the ν2 band suggests that it has predissociation lifetime of less than 1 ps. © 1995 American Institute of Physics.