The actual transient characteristics of the effectiveness factor for reactions in porous catalyst particles were considered in a model for kinetic studies in gradientless batch reactors. The transient effectiveness factor depends both on the Thiele modulus and the system's adsorption capacity, a function of the catalyst mass. After the injection of reactant, the transient effectiveness factor reaches a stable value, which is always larger than the conventional factor based on the assumption of steady state in the concentrations in the particle. The steady state approach on classical pseudo-homogeneous models can be used if the adsorption capacity of the system is low. A new pseudo-homogeneous model, where the accumulation of mass in the particle and the transient effectiveness factor approach are considered, extend this limit to systems with medium or high adsorption capacity. The usual interpretation of the Weisz-Prater parameter was reformulated based on this approach, showing that kinetic constants can be determined precisely if the transient effectiveness factor is employed, thus avoiding the errors induced by the accumulation of mass in the catalyst particles. Proper experimental conditions for kinetic studies were suggested. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.