In this paper, the homotopy analysis method (HAM), one of the most effective method, is applied to obtain the approximate solution of the nonlinear model of diffusion and reaction in catalyst pellets for the case of nth-order reactions. The approximate analytical solution obtained by HAM logically contains the solution obtained with Adomian decomposition method. The homotopy analysis method contains the auxiliary parameter h, which provides us with a simple way to adjust and control the convergence region of solution series. By suitable choice of the auxiliary parameter h, we can obtain reasonable solution for large Thiele modulus. (c) 2007 Elsevier B.V. All rights reserved.