In this contribution a mathematical model suitable to simulate cold start effects over metal-promoted zeolite SCR catalysts is developed and included into an existing SCR converter model. We start from a recently published microkinetic scheme, which describes the NO2 related SCR reactions at temperatures above about 100 C on the basis of a complex reaction network, taking into account NO2 storage with formation of nitrates and their subsequent decomposition/reactivity. In order to describe material and thermal cold start effects observed from room temperature we extend such a kinetic scheme to include the low-T NO oxidation to NO2, NO2 physisorption and H2O adsorption. The rate parameters are fitted to our data from lab-scale runs analyzing each individual step. We show that the resulting model can reproduce the transient effects related to cold start both at the lab scale and in engine test bench experiments. (C) 2014 Elsevier B.V. All rights reserved.