This article reports the bipolar and unipolar resistive-switching modes coexisting in TiN\HfO2\Pt systems. The unipolar switching characteristics exhibit progressive forming as well as gradual reset traces, while the set switching trace is abrupt. Based on our results, we propose a simple model based on the generation/recovery of oxygen-vacancy defects at the Pt interface during switching, in accordance with the gradual character of the resistance change. We show that the programming of the intermediate resistance levels, corresponding to only a partial recovery of the vacancy defects, leads to weaker retention properties. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3373529] All rights reserved.