We report mid-IR predissociation spectra of the I-. HCOOH . Ar-m(m=1-4) ion-acid complexes. The spectra are consistent with a planar structure where both hydrogens are engaged in ionic H bonds. Upon binding to the ion, the OH stretching fundamental displays a much more dramatic redshift (792 cm(-1)) than that of the CH stretch (99 cm(-1)), giving rise to a complex series of bands in the 2750-2950 cm(-1) region. The contributions of the CH and OH stretches to the spectrum are isolated by recording spectra of the I-. DCOOH and I-. HCOOD species, which reveal that the OH stretching vibration is accompanied by combination bands involving soft modes while the CH stretch spectrum is dominated by a single feature. Some of the complexity in the I-. HCOOH spectrum arises from a strong Fermi resonance interaction between the upsilon =1 level of the OH stretch and an overtone or combination band involving CH motion. We compare this behavior to that of the previously reported I-. CH3OH and I-.H2O complexes.