Using Fourier transform infrared spectroscopy (FTIR), the viscosity of zinc 2-ethylhexanoate liquid has been found to correlate with the intensity of an asymmetric COO stretching resonance at 1632 cm(-1). This is consistent with the presence of the zinc carboxylate polymer, catena-2-ethylhexanato-(O,O')-tri-mu-2-ethylhexanato-(O,O')-dizinc(II) as the origin of the viscosity, a conclusion that is further supported by theoretical predictions. Density functional theory has been used to assign the IR spectra of the zinc carboxylate dimer, catena-2-ethylhexanato-(O,O')-di-(tri-mu-2-ethylhexanato-(O,O')-dizinc(I I)-formic) acid, and the model of the molecular liquid, mu-4-oxo-hexakis-(mu-2-ethylhexanato)-tetrazinc(II). The predicted spectra indicate that the decreased symmetry of the polymer relative to the zinc 2-ethylhexanoate liquid increases the intensity of the asymmetric carboxylate stretch at 1632 cm(-1) and leads to the observed correlation.