The challenges of future regarding the energy supply are linked to the limitation of fossil fuels, the avoidance of climatic relevant gases, and the worldwide increasing demand for energy. Therefore, the future energy supply is characterized by the increase of renewable energy: sun, water, wind and biomass. Entrained flow gasification of biomass is promising, since it is a highly efficient and flexible process. Low-grade fuels are chemically transformed at high temperatures (> 1200 degrees C) and pressures (up to 80 bar) into synthetic fuels. The inorganics of the fuel are converted into a slag, which forms a layer in the reactor. The thermophysical and chemical properties of the slag are defining the conditions and limitations of the gasification process. In this study, the fundamental thermophysical properties of bioliq slags are determined, in order to describe the heat transfer and the flow of the slag across the reactor by CDF-modelling of the bioliq-gasifier within the HVIGasTech project. By using a high temperature viscometer the viscosity and flow behavior of the slag were determined. In addition, the density and surface tension were measured by the sessile drop method. Thermometric and calorimetric methods were used to provide a detailed view onto the slags heat capacity and phase transitions. Thermodynamic calculations using FactSage and an in-house developed thermodynamic database for available solution phases and compounds were performed to obtain information about the ash-slag transformation. (C) 2017 Elsevier Ltd. All rights reserved.