Hydrogenated amorphous carbon (a-C:H) thin films have been deposited from a mixture of methane and hydrogen using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) technique. The films were prepared over a wide range of deposition pressure under a constant microwave power of 200 W, and the rf induced de bias voltage was maintained constant at -100 V by varying the applied rf power. It was found that changing the deposition pressure in the ECR-CVD process has a profound effect on the properties of the films deposited, as evidenced from their Raman scattering and optical absorption results. The hardness of the films was found to increase remarkably with pressure, whereas the number of bonded hydrogen as deduced from the IR absorption showed only a weak dependence. A hardness of up to 18 GPa was measured for films deposited at 36 mTorr. The sp(3) content in the films was also observed to increase with the deposition pressure as deduced from their Raman spectra. The results can be attributed to the efficient excitation of the plasma by the microwave power under the ECR condition, which at increasing pressure leads to a dense plasma and a high ion flux reaching the substrates The high concentration of atomic hydrogen present in the plasma is also found to be critical in determining the structural properties of the films.