The model of steady state diffusion and reaction in a catalyst pellet where a single reaction takes place is analyzed with the scope to predict effectiveness factor through a very simple and practical procedure. Non-linear usual reaction kinetics are used to investigate the agreement among exact or numerical predictions with approximate results. In all cases studied maximum deviations in the whole range of 0 values are below 4% which turns the procedure attractive and useful. To perform numerical integration of the classical non-linear diffusion and reaction differential equation a new procedure was used that avoid usual instabilities or the introduction of spline collocation methods when concentration profiles are very steep. Concentration resulting profiles are compared with those generated by an early expression proposed by the authors. (c) 2004 Elsevier B.V. All rights reserved.