Catalytic activity of several state-of-the art Pt/Pd diesel oxidation catalysts exhibited large hysteresis in the reaction of NO oxidation to NO2: higher catalytic activity was observed after approaching isothermal points from low temperatures, compared to approaching them from high temperatures. The phenomenon was shown not to be an artifact of a particular experimental system or method. A set of experiments with varied duration of isothermal segments, up to 15 h, showed the continued slow drift of the activity in the direction of convergence. This indicates that the hysteresis is not a product of multiple steady-states in the catalyst activity, but rather a reflection of slow changes in the state of the catalyst surface, presumably related to the oxidation and reduction of the precious metal. This assumption was confirmed by a set of reductive and oxidative pre-treatments of the catalyst, which have quantitatively accounted for the hysteretic effect. Remarkably, the extent of the hysteresis was strongly affected by the amount and nature of sulfur species on the catalyst surface. This observation provided additional insights into the nature of the observed phenomenon. (C) 2015 Elsevier B.V. All rights reserved.