The radiopacity of 33 commercially available composite materials was measured in terms of their equivalent thickness of an aluminium alloy/unit thickness of material. The theoretical significance of this definition was discussed. The radiopacities varied by more than an order of magnitude. Although nearly 30% of the materials have radiopacities less than [human] dentine, all materials claimed to be radiopaque were more so than dentine. The influence of operating potential, specimen thickness, X-ray generator type, film type, filtration and X-ray dose were investigated. The former 2 factors had a large influence on the radiopacity.