T he problem of finding a one-dimensional (I D) model to approximate the behaviour of actual three-dimensional (3D) catalyst pellets is undertaken. It is shown that the I D model proposed by Burghardt and Kubaczka (Chem Eng Proc 35: 65-74, 1996), called here the generalized cylinder (GC) model, is most suitable for this purpose, provided that its main parameter (the shape power sigma) is fitted to the behaviour of the actual pellet at low reaction rates. Calculations from the GC model are expected to be precise at around 1% for most geometrical cases of practical interest. The evaluation of sigma for a given pellet geometry involves the solution of a Poisson equation. An approximate method that greatly simplifies this task for finite cylinders of any cross-section shape is developed. The procedure assumes knowledge of a just for the cross-section (at most, a 2D problem). This is readily available for some practical cases, but if not, a suitable numerical procedure based on the boundary element method (BEM) is proposed. BEM is also suitable for the general 3D case.