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Infrared spectra of monomeric oxalic acid $-h2,-hd,-d2$ have been observed using neon matrix-isolation spectroscopy. In addition, spectra of oxalic acid $-h2$ vapor were obtained using a heated absorption cell with a ten meter pathlength. All infrared active fundamentals were observed for oxalic acid $-h2$ and $-d2$ except the low frequency torsion. All oxalic acid -hd modes are infrared active but few fundamentals correlating with the Raman modes of the symmetric monomers could be observed. The spectra are interpreted in terms of the intramolecularly hydrogen bonded structure and a force constant analysis based upon the new data will be presented. Attempts to label an appreciable amount of oxalic acid with $\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}O$ by oxygen exchange with labeled water in very concentrated (acidic) solution failed. Similar exchange experiments with other carboxylic acids have resulted in excellent matrix-isolation spectra. Oxalic acid is known to readily exchange oxygen atoms with water in dilute solution and its failure to do so in concentrated solution presents an interesting problem in its own right concerning the structure of this liquid phase.""