The effects of preferential diffusion of hydrogen in downstream interacting counterflow H-2-air and CO-flames were investigated using numerical simulations. The global strain rate was varied in the range 305917 s(-1), where the upper bound of this range corresponds to the flame-stretch limit. Preferential diffusion of hydrogen was studied by comparing flame structures with a mixed average diffusivity with those where the diffusivities of H, H-2 and N-2 were assumed to be equal. Flame stability diagrams are presented, which show the mapping of the limits of the concentrations of H-2 and CO as a function of the strain rate. The main oxidation route for CO is CO + O-2 -> CO2 + O, which is characterized by relatively slow chemical kinetics; however, a much faster route, namely CO + OH? CO2 + H, can be made available, provided that hydrogen from the H-2-air flame can penetrate the CO-air flame. This modifies the flame characteristics in the downstream interaction between the H2-air and CO-air flames, and can cause the interaction characteristics at the rich and lean extinction boundaries not to depend on the Lewis number of the deficient reactant, but rather to exhibit anomalous behavior with a much stronger dependence on the interaction between the two flames. Such anomalous behavior includes a partial opening of the upper lean extinction boundary in the interaction between a lean H-2-air flame and a lean CO-air flame, as well as the formation of two islands of flame sustainability in a partially premixed configuration with a rich H-2-air flame and a lean CO-air flame. At large strain rates, there are two conditions where the flame can survive, depending on the nature of the interaction between the two flames. Furthermore, the preferential diffusion of hydrogen extends both the lean and the rich extinction boundaries. (C) 2013 Elsevier Ltd. All rights reserved.