The effect of the incorporation of oxygen and nitrogen on the structure of films obtained by PECVD of hexamethyldisiloxane (HMDSO)-He-N-2 and HMDSO-He-O-2 mixtures is investigated using infrared and Raman spectroscopies. From transmittance spectra of films deposited onto single-crystal KBr disks, the transverse optical (TO) and longitudinal optical (LO) functions in the mid-infrared region were calculated. To correlate structural aspects with the observed LO-TO splittings, an identification analysis of functional group based on the infrared and Raman literature was made. It was concluded that the structure of the films deposited from HMDSO-He-O-2 discharges was strongly dependent on the proportion of oxygen in the gas feed. In the absence of oxygen, i.e., for a discharge of a HMDSO-He mixture, the resulting film consisted of a network of interconnected siloxane and carbosilane units. Addition of O-2 precluded the formation of methylene bridges and induced the formation of a material enriched with Si-O-Si groups. Films formed from the HMDSO-He-N-2 plasmas, on the other hand, consisted mainly of interconnected siloxane and carbosilane units in addition to a small quantity of silazane units. On the basis of these results, we propose an interpretation for the variation of the LO-TO splitting amplitude for the asymmetrical stretching mode (AS1) of Si-O in Si-O-Si groups as a function of the oxygen or nitrogen incorporation into the films.