A linear approximation method to estimate effectiveness factors for isothermal catalytic pellets has been developed. The method is based on the Taylor series expansion of the reaction rate expression above the surface particle concentration for the key component. The application of the method to reaction systems with nonlinear kinetics yields simple analytic expressions for the effectiveness factor equivalent to that found for first order irreversible reaction. The effectiveness Factor expressions obtained depend of one parameter analogous to the Thiele modulus. In this work we report the results obtained for a Monod type kinetic equation. The effectiveness factors predicted are compared with those obtained from the numerical solution of the exact boundary value problem. The error is negligible at low Thiele moduli and as this parameter is increased the error grows, reaching an asymptotic value at high Thiele moduli. For the example presented, the error is below 10%. The available results indicate that nonlinear cases such as Langmuir-Hinshelwood type of equations can be treated successfully using the method developed.