Evaluation of thermal-recovery processes requires relative permeability functions, how such functions change with temperature, as well as the evolution of rock permeability and porosity as floods progress. There are significant challenges associated with estimating relative permeability, and its temperature dependence, from laboratory data including the accuracy of data and generalized assumptions in the interpretation technique. Direct measurements of relative permeability curves through the steady-state method are ideal and scarce, in particular for low-permeability systems. This study reports the results of a long-term study to measure the steady-state oil/water relative permeability as a function of temperature of a core sample from a low-permeability, siliceous shale, diatomite reservoir. All tests were isothermal, carried out at temperatures ranging from 45 to 230 °C (113 °F – 446 °F). In-situ phase saturation was monitored using an X-ray CT scanner. Results obtained show a systematic shift towards increased water wettability with increasing temperature. Dependence of endpoint phase saturation and relative permeability with temperature is apparent from these measurements thereby confirming the transition to water-wetness with increasing temperature previously derived from spontaneous imbibition experiments on previous diatomite samples. A post-mortem analysis of sample porosity distribution and rock quality reveals the evolution of additional petrophysical attributes as a result of two years of thermal flooding.