Methane production yields due to sub-eV H° impact on carbon are in the 10−3 – 10−4 CH4/H° range, i.e., about two orders of magnitude less than CH4/H+ yields for 0.1 – 100 keV H+ ions. Two macroscopic states of “reactivity” were identified for carbon: an “activated” state characterized by a CH4 yield vs. sample temperature curve with a maximum at 700–850K, and a “deactivated” state characterized by a monotonically decreasing yield as a function of temperature. Regarding the retention of sub-eV H° and D° in carbon, our results differ from previously published results. We have observed lower levels of trapped H° (∼1015 H°/cm2), with an apparent trend for saturation, at incident fluences of >2×1019 H°/cm2. Strong synergistic effects have been reported for combined sub-eV H°/5 keV Ar+ impact, while it appears that “insignificant” synergism exists for combined sub-eV H°/e− impact.