This work deals with the analysis of a kinetic model of two parallel, first order, irreversible reactions that include a second order inhibition term in one of them (i.e. A + b(1)B --> Ck1CA/(1+KCA)2,A + b(2)B --> D-k2CA). The continuity differential equation taking account of isothermal diffusion and reaction of A in a spherical catalyst pellet, was formulated. The numerical solution of the latter equation yielded useful results related to the variation of the effectiveness factor (eta) and the selectivity (S) (S = [k(1)C(A)/(1 + KCA)(2)]/[k(2)C(A) + k(1)C(A)/(1 + KCA)(2)]) for the desired reaction (i.e A + b(1)B --> C) versus the Thiele Modulus (phi). Parametric studies involved the investigation of the effects of k = k(2)/k(1), the ratio of the intrinsic specific reaction rate constants, and the inhibition strength factor (i.e. KCA), upon the eta vs. phi and the S vs. phi curves. The eta vs. phi curve turns faster towards lower eta values for high k values, especially at high inhibition KCA values. Intraparticle diffusion imparts a pronounced effect upon selectivity, a fact contrasting markedly from the standard case of two parallel reactions without inhibition, where selectivity is independent of diffusion resistance. The S vs. phi curves show a step increase occurring at specified values of phi that increase at high inhibition strengths. The relative selectivity S '(= S/S-0) vs. phi curves (where S-0 is the selectivity for reactant concentration at catalyst surface) increases monotonically with k and KCA values and go through a maximum for high phi values.