Electron paramagnetic resonance (EPR) measurements have been made at X-band (≈ 9.5 GHz) and W-band (≈ 95 GHz) of a-C:H films on (1 0 0) silicon substrates; the sample temperature was varied in the range 5-300 K. Two types of film were examined. The first type are amorphous hydrogenated carbon (a-C:H) films grown by plasma enhanced chemical vapour deposition (PECVD) with negative self bias voltages in the approximate range 100-500 V. The second type were initially highly polymeric-like a-C:H films grown on Si placed on the earthed electrode of a PECVD system but were subsequently implanted with either 6 × 1015 cm-2 B+ or 2 × 1016 cm-2 B+ ions. At both X- and W-band and throughout the temperature range 5-300 K the EPR signal of the carbon unpaired electrons consists of a single symmetric line with g = 2.0026 ± 0.0002. As the temperature is lowered, several samples develop a dependence on sample orientation of the external field required for resonance. This anisotropy is explained in terms of the demagnetising fields more usually encountered in ferromagnetic resonance.