There is a notion that in carbonaceous materials the sp(3) hybridised fraction determines the mechanical properties, while the sp(2) fraction controls the electro-optical properties. We investigated a narrow range of a-C: H films synthesised in an open hydrocarbon plasma source. The a-C:H films displayed unambiguous variation in mechanical properties as measured by nanoindentation measurements. However the proportion of sp(3) constituent, determined using X-ray C-1s core-level photoelectron spectroscopy and Fourier transform infrared spectroscopy, was found to be essentially the same for all studied samples. The Tauc gap was found to be controlled by the perturbation of the pi tail states of sp(2) phase and its magnitude varied inversely with a surface conduction gap determined by scanning tunnelling microscopy measurements. These results indicate that mechanical properties of a-C:H films are also influenced by the degree of disorder of the sp(2) fraction, as characterised by bond saturation and symmetry. Crown Copyright (C) 2007 Published by Elsevier B.V. All rights reserved.