Effective medium theory (EMT) is a semiempirical approach developed to predict the response properties of composites. In particular, EMT has been applied to the study of rough metal surfaces that can enhance the absorption of electromagnetic radiation in the infrared [surface-enhanced infrared absorption (SEIRA) or in the visible (surface-enhanced visible]. The application of EMT provides a formalism to simulate the effective dielectric function of the inhomogeneous medium, for instance, of metal-organic thin films. The EMT is widely applied when the dimensions of the inhomogeneties (granular components in the mixed film) are smaller than the wavelength of the incident radiation. The computational approach to SEIRA using EMT, and the experimental SEIRA results for pentacene are presented here. First, enhancement factors are calculated using EMT method for pentacene on Ag, Cu, and Sn. Vibrational intensities for each symmetry species are obtained using DFT B3LYP at the 6-31G(d) level of theory. Second, from the set of experimental data provided by reflection- absorption infrared (RAIRS) and transmission FT-IR spectra of a 15 nm pentacene film evaporated onto reflecting and IR transparent support substrates, respectively, the molecular orientation and the interpretation of the observed spectra in conformity with the surface selection rules was extracted. The interpretation of the pentacene SEIRA spectra of a 15 nm pentacene film on silver and tin islands is presented and its compliance with surface selection rules is discussed.