The problem of calculating the effectiveness factor for a porous slab of catalyst pellet under non-isothermal conditions has been revisited. An exact formal analytical solution was obtained for a nth-order (integer n greater than or equal to 0), exothermic and irreversible chemical reaction. Numerical calculations where performed, and an analytical formula was obtained for the very fast reaction limit. In addition, a fairly simple formula, which was called the low beta approximation, was developed and tested to be valid for low values of the thermicity group beta less than or equal to 0.1. (C) 2004 Elsevier Ltd. All rights reserved.