The molar quantity of adsorbed CO and H2 present on the surface of a mixed CuO x -CeO2 catalyst during CO preferential oxidation in H2 at 353 K was quantified using both the reactive titration method and the steady-state isotopic-transient kinetic analysis (SSITKA). For the reactive titration method, either CO or H2 was replaced by He during steady state reaction while monitoring the residual transient product formation of CO2 or H2O produced from the surface adsorbed CO or H2 via catalytic oxidations. For SSITKA, 12CO was replaced by 13CO during steady state reaction while monitoring the transient product formation of 12CO2. The amount of adsorbed reactive CO increases with increasing CO partial pressure or decreasing H2 partial pressure, while the amount of reactive H2 decreases with increasing CO partial pressure or decreasing H2 partial pressure, showing that the adsorbed CO and H2 compete for active redox sites and prohibit the other’s adsorption. Using reactive CO and H2 amounts, two models of coverage were defined with trends providing insight into the competitive redox mechanism between adsorbed CO and H2. CO oxidation is kinetically preferred over CuO x -CeO2, and the relative CO to H2 coverage is shown to be the determiner for CO2 selectivity. This new depiction of selectivity parameters provides a useful principle for the design of selective PROX catalysts.