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->k 1 C A /(1+KC A ) 2 C,A+b 2 B->k 2 C A D). 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 (η) and the selectivity (S)(S=[k 1 C A /(1+KC A ) 2 ]/[k 2 C A +k 1 C A /(1+KC A ) 2 ]) for the desired reaction (i.e A+b 1 B->C) versus the Thiele Modulus ( ). 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. KC A ), upon the η vs. and the S vs. curves. The η vs. curve turns faster towards lower η values for high k values, especially at high inhibition KC A 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. curves show a step increase occurring at specified values of that increase at high inhibition strengths. The relative selectivity S'(=S/S 0 ) vs. curves (where S 0 is the selectivity for reactant concentration at catalyst surface) increases monotonically with k and KC A values and go through a maximum for high values.