We report results obtained from optical absorption studies carried out on amorphous silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane plasma. The influence of the film thickness was studied on the two series of samples deposited from undiluted silane and under moderate hydrogen dilution of silane. Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band-gap energies E-g, the B factors of the Tauc plots, the iso-energy values E-04 (energy at which the absorption coefficient is equal to 10(4) cm(-1)). The results were correlated with volume fractions of the amorphous phase and voids and with the film thickness. (C) 2010 Elsevier B.V. All rights reserved.