Some form of hydrogenated amorphous carbon (HAC) is widely regarded as a likely component of dust in the diffuse interstellar medium of the Galaxy. The most direct observational evidence is the ∼2950 cm −1 C-H stretch feature seen in absorption along heavily attenuated lines of sight. We have used the measured properties of one well-characterized HAC sample, together with well-established correlations between the structural, optical, and infrared (IR) characteristics of HAC materials in general, to estimate the amount of carbon relative to hydrogen that must be bound in this particular solid in interstellar space. The solution to this problem remains uncertain because of the strong dependence of the ∼ 2950 cm −1 mass-absorption coefficient upon the H/C ratio in HAC materials. We estimate that about 80 ppM carbon relative to hydrogen is locked up in interstellar HAC grains with H/C ≃ 0.5. A H/C ratio lower than this value would require higher carbon depletions and result in a ∼ 2950 cm −1 absorption profile inconsistent with observations. Substantially higher H/C ratios, which would imply carbon depletions as low as 20 ppM , would lead to materials exhibiting efficient blue-green photoluminescence, which is not observed. Combined with observations of the gas-phase interstellar abundance of carbon and the revised interstellar reference abundances of refractory elements, this result implies that the bulk of solid carbon in interstellar space is in the form of HAC. In addition to a careful analysis of the IR properties of HAC, we present a summary of the UV/visible optical properties, including the complex index of refraction, measured for one particularly well-characterized HAC sample that is shown to have an IR absorption spectrum which is quantitatively consistent with the latest IR observations of the diffuse interstellar medium.