Fourier transform infrared spectroscopic measurements have been made on monolayer samples of cadmium arachidate in order to determine orientation and molecular packing on the surface. This was accomplished by using both grazing angle reflection methods, where the polarization of the infrared radiation is very close to being perpendicular to the surface, and transmission methods, where the incident optical electric field is polarized parallel to the surface. Hence these two methods are sensitive to molecular vibrations whose change in dipole moment lies along different directions. Our results showed that independent of the substrate, silver for the reflection experiments and silver bromide for the transmission experiments, the chains of the fatty acid salt (no evidence for any free acid was found) are oriented within a few degrees of the normal to the surface of the substrate. From a detailed analysis of the observed vibrational bands in the two orientations, combined with the known literature values and assignments, we were able to make a "complete assignment" of the observed bands. Our experimental results and conclusions will be presented. © 1983.