This paper describes an approximation method for the estimation of effectiveness factors in porous catalysts that have reactions with non-linear kinetic models. The reaction rate expression is expressed as a combination of two asymptotic solutions based on a local concentration, leading to an approximate rate function containing four parameters. The effectiveness factors computed with the approximated rate are shown to be close to the values calculated from the real rate function for a variety of non-linear LHHW type models. The advantage of the approximate method is that a given set of four parameters, which can easily be calculated, uniquely identifies an effectiveness factor. This observation enables the construction of a table from which the effectiveness factor can be readily extracted during reactor simulation, thus resulting in a decrease in computational effort. (c) 2007 Elsevier Ltd. All rights reserved.