Hydrogenated amorphous carbon films were prepared by plasma enhanced chemical vapour deposition (PECVD) of methane-argon (5%) gas mixtures at low pressure, in a dual electron cyclotron resonance (ECR)-rf glow discharge. Optical transmission spectroscopy, Raman spectroscopy, elastic recoil detection analysis (ERDA) and infra-red (IR) absorption spectra were combined to examine the relationship between the local microstructure (C-H and C-C bonds) and the optical properties at different negative bias voltage. The amount of bonded H, obtained by IR spectra, is higher in series deposited at low bias voltage, in qualitative agreement with the ERDA results. In ah cases, most of the incorporated H is bonded to sp(3)C sites, with a predominance of CH3 methyl groups. These results are also consistent with the Raman spectroscopy measurements. The positions, widths, and relative intensities of the two characteristic features, the D and G peaks are found to vary systematically with deposition conditions and film properties. The series prepared at high bias voltage shows a higher disorder (D) over the graphitic (G) band ratio, which also indicates a structure with a high disorder form of Csp(2) sites.