Periodic density functional theory (revised PBE; RPBE) and ab initio atomistic thermodynamics were used to assess the dissociative adsorption of H2O and CO2 on a (5 x 5) Ru(0001) surface model. For H2O dissociation on clean Ru(0001) [H(2)0 -> OH + H -> O + 2H], the first step has lower barrier than the second step (0.75 vs. 0.80 eV). On O pre-covered Ru(0001), H2O dissociation [H2O + O -> 2OH] has lower barrier (0.62 eV) and is more exothermic (-0.29 vs. -0.15 eV). CO2 dissociation on clean Ru(0001) [CO2 -> CO + O] needs low barrier (0.23 eV) and is highly exothermic by 1.47 eV. In turn, CO2 formation from surface O removal by CO has barrier of 1.70 eV, in agreement with the experimentally detected 1.80 +/- 0.15 eV. That the computed CO desorption energy (1.55 eV) is smaller than CO oxidation barrier is in agreement with the experimental finding. The computed desorption temperatures of H2O, CO and CO2 under ultrahigh vacuum conditions agree perfectly with the experiments. In addition, high oxygen and OH pre-coverage do not significantly affect the energetics of the dissociation of H2O and CO2. The excellent agreement between theory and experiment confirms the applied models and methods, and in turn, gives the reliability and confidence about the further predicted results.