The ground state of He–HCO⁺ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v₁ C–H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He–HCO⁺ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.