Infrared spectra of mass-selected Cl⁻–C₂H₄ and Br⁻–C₂H₄ complexes are recorded in the vicinity of the ethylene CH stretching vibrations (2700–3300 cm⁻¹) using vibrational predissociation spectroscopy. Spectra of both complexes exhibit 6 prominent peaks in the CH stretch region. Comparison with calculated frequencies reveal that the 4 higher frequency bands are associated with CH stretching modes of the C₂H₄ subunit, while the 2 weaker bands are assigned as overtone or combinations bands gaining intensity through interaction with the CH stretches. Ab initio calculations at the MP2/aug-cc-pVDZ level suggest that C₂H₄ preferentially forms a single linear H-bond with Cl⁻ and Br⁻ although a planar bifurcated configuration lies only slightly higher in energy (by 110 and 16 cm⁻¹, respectively). One-dimensional potential energy curves describing the in-plane intermolecular bending motion are developed which are used to determine the corresponding vibrational energies and wavefunctions. Experimental and theoretical results suggest that in their ground vibrational state the Cl⁻-C₂H₄ and Br⁻-C₂H₄ complexes are localized in the single H-bonded configuration, but that with the addition of modest amounts of internal energy, the in-plane bending wavefunction also has significant amplitude in the bifurcated structure.