While there is certainly a higher degree of error cancellation for pure surface elementary steps than adsorption reactions, error cancellation is not automatically guaranteed, and the degree of cancellation can vary widely depending on the nature of the adsorbed species. To demonstrate, we show the computed reaction enthalpy for a set of surface reactions using the BEEF-vdW and RPBE functional compared to the experimental values in Fig. . The experimental ∆ f H of the adsorbates were derived from the adsorption enthalpies reported by Silbaugh and Campbell . While most of the predicted reaction enthalpies are within ±25 kJ mol -1 , there are deviations of up to 90 kJ mol -1 from the experiment. Additionally, results from the different functionals can vary substantially. For example, the reaction enthalpy for the dissociation of *OH determined from the RPBE xc-functional agrees within chemical accuracy with the experiments, while the BEEF-vdW value deviates by 35 kJ mol -1 . This large deviation and sensitivity to functional choice indicates a low degree of error cancellation. There are also reactions for which both functionals give similar results, indicating a higher degree of error cancellation since the specific errors of each xc functional are canceled out.