The cis and trans conformers of benzaldehydes substituted in the o- or m-positions by Cl or CN can be differentiated by IR spectroscopy in Argon matrices. Partial photochemical rotamerization allows assignment of the IR bands and establishment of their relative intensities in pairs of rotamers, which permits, in turn, quantitation of equilibrium compositions prior to photolysis. The observed spectra and equilibrium compositions are in very good agreement with the predictions of vibrational spectra and free energy differences from B3LYP/6-31G calculations. The present work represents the first attempt to quantitate the small contributions that the cis rotamers make to the room-temperature equilibrium compositions of the two o-substituted benzaldehydes. The results for the m-substituted derivatives are compared to earlier estimates based on other methods.