FTIR phase-modulated ellipsometry has been used to characterise hydrogenated amorphous silicon (a-Si:H) thin films, deposited by PECVD, on substrates covered by a discontinuous Au film. The metallic islands generate surface-enhanced infrared absorption (SEIRA) in the deposited a-Si:H material. The ellipsometric infrared spectra of the substrate /(Au+void) and substrate /(Au+aSi:H)/a-Si:H structures were simulated assuming a multilayer model and using the Bruggeman effective medium approximation (EMA) to account for the metallic island shape distribution and the uniaxial anisotropy generated by the islands. TEM results were used to avoid parameter correlation in the fitting procedure. The a-Si:H material showed absorption bands at 2100, 2140 and 2250 cm(-1) that could be associated with the Si-H-2, OSi2-SiH and O-3-SiH stretching vibrations, respectively. The intensity of these absorption bands was enhanced up to a factor of five due to the presence of the Au island film.