Two a-C:H samples were prepared using a fast-atom deposition system from acetylene and an acetylene/hydrogen gas mixture. Their structure was investigated using neutron and x-ray diffraction and infrared spectroscopy measurements. Compositional analysis shows that a 1:1 mixture results in a change from to a-, i.e. has a very small effect on the composition. The diffraction data also show that the addition of hydrogen to the precursor gas has no significant effect on the average bond distances and angles but shows a small change in the H-C-H and C-C-H correlations between the two samples. However, the infrared data show that there are significant changes in the bonding of hydrogen within the sample - changes which do not affect the average network structure. We observe a decrease in the amount of and groups, and an increase in the fraction of and CH groups, with the formation of a second CH bonding environment in the hydrogen-diluted sample. Therefore, in addition to providing useful structural information on these a-C:H samples, this set of experiments illustrates very well the complementary nature of the data from diffraction and spectroscopic techniques.