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The diffusion and reaction problem in catalytic pellets of any shape subject to diffusive transport limitations is undertaken in this contribution. Effective reaction rates in three-dimensional (3D) catalysts can be evaluated through a series solution written in terms of powers of (1/Φ) when strong diffusion limitations are present. In a recent paper, Keegan et al. [Chem. Eng. J. 2005, 110, 41] have clearly demonstrated for smooth catalysts that the second-order term [in (1/Φ) 2 ] depends essentially on the shape of the pellet. In this context, the purpose of this paper is to develop expressions of the second-order term for two-dimensional (2D) or 3D catalytic bodies showing edges. While the first-order term allows definition of the proper size of a catalyst, the second-order term provides a characterization for the shape of the catalyst. The possibility of using this characterization of catalyst shape in a geometrical one-dimensional (1D) model to approximate the behavior of given 2D or 3D pellets is analyzed. Also, the direct use of the two-term truncated series for complementing the numerical evaluation of the conservation equation is described.