A novel method, using isotope exchange is discussed for determining the elementary steps in surface reactions. The method is presented using oxygen exchange with oxides as a working example. We apply the method to a series of possible exchange reactions and show how to differentiate between them. The slow step and possible fast reversible steps preceding the slow one can be identified. We pay attention to the role of oxygen vacancies in the dissociation of oxygen molecules on oxides. The experimental requirements are that the amount of (OO)-O-16-O-18 molecules desorbed from the oxide is monitored with time while two parameters are changed: the oxygen partial pressure in the experiment and the concentration of acceptors in the oxide. The most suitable experimental method to provide the necessary data is the pulse-response (OO)-O-16-O-18 isotope exchange (PIE) technique. (C) 2015 Elsevier B.V. All rights reserved.