Many writers model gaseous mass transfer to, and surface reaction on (or with), a solid sphere without acknowledging that the reaction stoichiometry contradicts the equimolar counterdiffusion (EMCD) assumption that they use in their mass transfer analysis. The flux ratio correction factor which ought to be applied to the commonly used EMCD calculation is presented. Analysis and sample calculations for fast and slow reactions show that errors of more than 10%, and up to 85%, might arise in practice if the necessary correction is overlooked. Significant errors may often be avoided by using a dilute reactor feed: analysis and sample calculations reveal that in the most extreme, but realistic, case considered, the reactor feed must contain less than 3.7% mole fraction of reactant to ensure an error of less than 10% in predicted reaction rate.