In ilmenite smelting furnaces, a freeze lining of solidified slag is used to protect the furnace refractories against the aggressive titanium slag. Freeze lining thickness cannot be measured directly due to harshness of conditions inside the process, thus process modelling is required. Several parameters influence the thickness of the freeze-lining, one of them being thermal conductivity of the frozen slag. However, there is a lack of thermal conductivity values for high titanium slags -especially as a function of temperature. In this study, thermal conductivity of three titanium slag samples and an additional sample of freeze-lining was measured from room temperature to 1100/1400 °C with the laser flash analysis method. In addition, thermal expansion and microstructures of the samples were studied to provide an extensive understanding of how microstructure will affect thermal conductivity. The thermal conductivity of the slag samples was found to increase from 1.2 to a maximum of 2.4 W/(m K) when increasing temperature from room temperature to 1100 °C. An additional experiment at 1400 °C showed that the thermal conductivity increased further as the temperature increased. The freeze-lining sample behaves differently, with conductivity being the highest at room temperature, 2.2 W/(m K).