Hydrogenated amorphous carbon (a-C:H) films were deposited from methane in a dual microwave electron cyclotron resonance (ECR) radio frequency (r.f.) plasma by applying an independently controlled r.f. substrate bias voltage. In situ real-time ellipsometry was used to monitor the evolution of the film growth during the deposition. The combined in situ/ex situ ellipsometry and Raman spectroscopy measurements show that the deposited film properties change from polymer-like to diamond-like depending on the presence of the applied bias voltage. The spectroscopic ellipsometry analysis exhibits a decrease in the optical bandgap energy and an increase in the sp(3)/sp(2) ratio with the r.f. bias voltage. Raman spectra have been analyzed using a four-Gaussian deconvolution procedure which allowed the assignment of the two D and G peaks as well as two additional bands at around 1170 and 1480 cm(-1) suggesting the presence of a diamond phase enhanced by the presence of the r.f. bias voltage.