The interaction of CO with the Ru(0001)(1 x 1)H surface has been studied by density functional theory (DFT) periodic calculations and molecular beam techniques. The hydrogen (1 x 1) phase induces an activation barrier for CO adsorption with a minimum barrier height of 25 kJ mol(-1). The barrier originates from the initial repulsive interaction between the CO-4 sigma and the Ru-d(3z2-r2) orbitals. Coadsorbed H also reduces the CO adsorption energy considerably and enhances the site preference of CO. On a Ru(0001)(1 x 1)H surface, CO adsorbs exclusively on the atop position.