Understanding the relative stability of CHx species on surfaces is necessary for mechanistic description of much important catalytic chemistry. Here, we experimentally quantify the barrier of the reaction CH2 -> CH + H on Ru(0001) in UHV and find an activation energy, 65 +/- 6 kJ/mol, that is >4x higher than previous computational results with 0, 1, or 2 coadsorbed H atoms per CH2, i.e. 16 kJ/mol. Employing density functional theory calculations, we show that this disagreement can be reconciled if 3 coadsorbed H atoms per CH2 are present in our experiment. We further demonstrate, by calculating the surface phase diagram for one carbon species on Ru(0001) as a function of H-2 chemical potential, that the additional hydrogen surface coverage requires non-equilibrium conditions. Such conditions may be important at the high temperatures and pressures of real catalytic systems. (C) 2014 Elsevier Inc. All rights reserved.