We have detected a polar conformer of oxalic acid and investigated its structure using Stark-modulated free jet microwave spectroscopy. By comparing observed and ab initio predicted spectroscopic rotational constants and deuteration isotopic substitution coordinates, the species has been identified with the cis, trans, trans conformer (cTt), in which one of the carboxyl hydrogen atoms is involved in an intramolecular hydrogen bond with the carbonyl oxygen of the other carboxyl group. Stark effect measurements yielded electric dipole moment values of mu(a), = 2.850(6) D; mu(b) = 1.148(3) D, mu(c), = 0.0 D (assumed), and hence mu(total) = 3.073(6) D. Ab initio calculations at the MP2/6-311++G(d,p) level predict and experiment confirms that no other polar conformers occur in detectable concentrations in the vapor at the experiment spectral search scan preexpansion temperature of 458 K. However, two undetectable species, one nonpolar (tTt), the other of low polarity (c"T"c, mu = 0.6 D) are predicted to be present in greater concentrations than the observed species. Predicted relative stabilities of the different conformers are appreciably affected by the inclusion of Gibbs free energy corrections via the vibrational partition function. Large amplitude anharmonic vibrations, for which numerical integration of the vibrational Schrodinger equation is required for obtaining the correction, have an important influence on predicting relative stabilities of the oxalic acid conformers.