Approximate analytical expressions of overall effectiveness factors for parallel catalytic reactions occurring in a spherical catalyst pellet with both internal and external mass transfer resistances are obtained after taking the information given by asymptotic solutions valid for small and large values of Thiele moduli. This approximate procedure is feasible for power law as well as Langmuir-Hinshelwood kinetics and the agreement between the approximation and the numerical results is good enough for practical purposes. Moreover, the method does not require any iterative or trial-and-error computation, and can be conveniently applied in most practical calculations for reactors giving reliable results.