Infrared spectra are reported for 24 isotopically labeled (D, 13C, 18O) formic acid monomers isolated in a neon matrix. Clear heavy atom isotope effects are observed for most of the fundamental vibrations and several of these are greater than 20 cm -1. Using 163 observed frequency values, F matrices with 17 elements (no OH or CH interaction constants) and with 28 elements (full F matrix) are presented for the a' symmetry block. Results for similar calculations using "harmonized" frequencies are presented. The normal coordinates of the low-frequency modes show extensive mixing of the internal coordinates, thus precluding a simple normal mode description. Fermi resonance interactions appear clearly in the matrix isolation data: one of these couples the carbonyl vibration with the DC wagging overtone of the DCOOH(D) molecules.