An analysis of the origin and passivation of interface states in (100)Si/SiOx/HfO2/TiN capacitor structures is presented. For high-k gate/metal gate capacitors which exhibit relatively high interface state densities (> 1x1011cm-2) the dominant interfacial defects are silicon dangling bond (Pbo) centres. For (100)Si/SiOx/HfO2/TiN capacitors which experience no high temperature thermal budget following HfO2/TiN gate formation (T<600{degree sign}C), the devices exhibit instabilities, where the interface state densities are modified during electrical measurements. The origin of this instability is studied. The response of the interface state density to rapid thermal annealing (30s) in N2 over the temperature range 600-900{degree sign}C is presented. In addition, results are presented for interface state passivation in forming gas (0.5H2/0.95N2) from 350-550{degree sign}C for (100)Si/SiOx/HfO2/TiN gate stacks with no post deposition annealing following TiN gate formation and for devices following a 900{degree sign}C, 30s N2 RTA.