Most of the bountiful literature on the calculation of effectiveness factors for catalyst particles deals with relatively simple reactions and kinetics. The performance of catalyst particles in the industrially important, modified Claus reaction introduces multiple reactions, sulphur vapour equilibria, non‐linear kinetics and limiting thermodynamic conversions into the calculation of local effectiveness factors. This analysis demonstrates a means for generating local effectiveness factors, during reactor design calculations by the use of a single calculated curve of versus a modified Thiele parameter for spherical particles.