The microstructural, optical, mechanical and electrical properties of diamond-like carbon films prepared by d.c. magnetron sputtering have been investigated as a function of argon pressure and substrate bias. The films were characterized by spectroscopic ellipsometry, scanning and transmission electron microscopy, electron diffraction, resistivity and hardness measurements. The pseudoband gap was found to rise and the hardness decrease with increasing pressure. Ellipsometric modelling indicated a change in the bonding configuration under the above conditions. This effect does not appear to be the result of a change in the energetics of the bombarding ions. Increasing substrate bias resulted in a densification of the films.