The problem of diffusion and reaction in catalyst pellets is considered for the case of nth order reactions. The Adomian decomposition method is used to solve the nonlinear model of diffusion and reaction and to obtain approximate solutions. The variation of reactant concentration in the catalyst pellet and the effectiveness factors are determined for second-, half-and first order reaction. The approximate analytical solutions obtained are compared with solutions obtained with a finite difference numerical method. In general, the Adomian polynomial method with three terms gives solutions comparable to the numerical procedure for Thiele modulus approximately up to 4.0.