An efficient method for computing approximate value of the effectiveness factor is presented. The method is developed for an arbitrary rate expression and for three representative catalyst shapes, namely, an infinite slab. an infinite cylinder and a sphere. In the method. two new asymptotic expansions for small and large Thiele moduli are developed and joined together at an intermediate value of the Thiele modulus, which can be calculated by a simple equation. The approximation is highly accurate for small and large Thiele moduli. and it has small or negligible errors for intermediate values of the Thiele modulus. As demonstrated with examples. the approximation method is fast, straight forward and is a dependable alternative to numerical methods for computing exact values of the effectiveness factor. (c) 2006 Elsevier Ltd. All rights reserved.