This paper reports on the influence of forming gas annealing (5%H2/95%N2) over the temperature range 350 °C-550 °C on the density of electrically active interface states in Si(100)/SiO2/HfO2/TiN gate stacks. Prior to forming gas annealing the distribution of interface states across the energy gap exhibits the electrical signature of the Pb0 dangling bond centre for the hydrogen free Si(100)/SiO2 interface. Forming gas annealing at 350 °C and 400 °C results in a reduction of the interface state density, with an increase in interface state density for forming gas anneals in the range 450 °C-550 °C. The effect of the cooling ambient for the forming gas anneal (N2 or H2/N2) is also reported.