Amorphous hydrogenated boron-carbon (a-B1-xCx:H) thin films were prepared by radio-frequency plasma deposition using CH4 and B2H6 diluted in H-2 as precursor gases. The composition and density were determined by ion-beam analysis. The densities of the a-B:H and a-C:H films are similar with rho congruent to 1.72 g cm(-3), a value typical of hard and dense a-B:H and a-C:H films. Ternary a-B1-xCx:H films are less dense, having a minimum density of rho congruent to 1.48 g cm(-3) at x = 0.71. Fourier transform infrared spectroscopy was used for qualitative and quantitative analysis of the bonding structures. The properties of films up to x approximate to 0.2 are comparable to those of pure a-B:H films. However, small carbon contents influence the film structure, mainly reducing the number of B-H-B bridge bonds, which are abundant in pure a-B:H films. Low film densities at x = 0.5 to 0.8 correlate with the maximum intensity of the extrinsic B-C vibrational mode around 1200 cm(-1) due to terminal -CH3, groups indicating a less cross-linked network structure. Modifications of the network are indicated for x > 0.7. Below, carbon is predominantly sp(3) hybridised and bonded between boron neighbours. Above x = 0.7 the network becomes carbon dominated with an increasing number of carbon-carbon bonds and an increasing fraction of sp(2) hybridised carbon was observed. The extrinsic B-C vibrational mode at 1200 cm(-1) and the intrinsic B-C network vibration at 1500 cm(-1) were quantified over the whole range of a-B1-xCx:H film compositions. The absorption constant of the extrinsic B-C vibrational mode at 1200 cm(-1) was estimated.