Abstract: A method based on the orthogonal collocation technique was given for calculating the effectiveness factors of bidisperse catalysts. The set of coupled second-order differential equations for modeling the system was successfully solved. Legendre and Jacobi polynomials chosen as trial functions, and different combination patterns of collocation points in pellet and in particles, were tested for their influence on the rate of convergence and the final solution. Effectiveness factors were obtained for reversible and irreversible reactions. Values of the calculated effectiveness factor of first-order irreversible reactions agree very well with those of the exact solution, while those of second-order irreversible reactions are fairJy close to the trial-and-error solutions by Jayaraman. Effectiveness factors for reactions of the Langmuir-Hinshelwood kinetics were also obtained.