The peak frequency, width, and shape of the transverse-optical (TO) and longitudinal-optical (LO) infrared absorption modes of silicon oxides (SiO2, SiOx), silicon nitrides (Si3N4, SiNx), silicon oxynitrides (SiOxNy), and other silicon compounds have often been connected with stress, stoichiometry, defects, structural order, and other properties of the layers. However, certain geometrical effects strongly influence the spectral response of the material independent of its physical or structural properties. The influence of layer thickness, multilayer configuration, substrate effects, angles, and polarization on the behavior of TO and LO bands are presented and discussed. Some corrections are suggested to reduce experimental error and for the reliable measurement of stress, composition, disorder, and structure.