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Infrared spectra are reported for 24 isotopically labelled (D, $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C$, $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O)$ formic acid monomers isolated in a neon matrix. Clear isotope effects are observed for most of the fundamental vibrations. Using 156 observed frequency values (no DC- stretches) all 28 force constants of the $a\prime$ symmetry block are retained and estimated. The normal coordinates frequently show appreciable mixing of the symmetry coordinates, e.g. the C-0 stretch and the COH angle deformation. Two clear Fermi resonance are found that shift fundamental frequencies of DCOOD species. These are the carbonyl stretch and the skeletal mode located near $1030cm-1$.