Non-stoichiometric reduction of Cerium Oxide IV (Ceria) has garnered interest for its potential as a redox agent in high temperature water and CO2 splitting cycles. State of the art design and characterisation of high temperature redox reactors employs numerical simulations to calculate the heat transfer processes and reaction extent taking place during Ceria reduction. The optical and radiative properties of Ceria play a crucial role during the reduction process, and vary non-linearly with reaction extent δ. This paper presents experimental measurements of the complex refrac- tive index of partially reduced Ceria in the range δ = 0.050 − 0.105 which is relevant for redox reactors. Ceria samples were partially reduced in a furnace and complex refractive indexes were obtained with an ellipsome- ter. Experimental results show that the complex refractive index of Ceria varies non-linearly in a way that significantly affects transient radiative heat transfer calculations of reduction extent in high temperature redox reactors.