Zero-order reactions occurring in porous catalysts have been either ignored or handled incorrectly in major textbooks. In this work, the long-held misconception of dc/dx = 0 at x = 0 (center of the catalyst) is corrected by placing c = 0 and dc/dx = 0 at a critical position x(c) for cases of Thiele modulus >1, where c is concentration and x is the distance measured from the center plane of a slab or the center of a cylinder or sphere. Exact solution for xc is presented for the slab geometry and numerical solutions for the cylindrical and spherical geometries. Effectiveness factors are also calculated. It will be shown that the condition of dc/dx = 0 at x = 0 yields erroneous results and the numerical scheme presented in this work accurately predicts xc and effectiveness factor for Thiele modulus >1.