The selectivity of two first-order, parallel reactions, one reversible and one irreversible, can be altered substantially by the presence of an intraparticle diffusion resistance. At very large resistances, the relative selectivity approaches an asymptotic limit which depends on the kinetic, transport and thermodynamic parameters of the reaction system. It is possible for the selectivity to the product of the reversible reaction to change sign as the pore diffusion resistance increases, i.e. there can be a net formation of that product at low resistances and a net consumption at high resistances. In some cases, the rate of consumption of the product of the reversible reaction can be so high that the ratio of product concentration to reactant concentration in the bulk fluid moves away from the equilibrium ratio. This behavior contrasts markedly with that of two irreversible, first-order reactions in parallel, where the selectivity is independent of the intraparticle diffusion resistance.