Pressurized entrained-flow gasification (PEFG) of straw biomass is currently being studied as a potentially sustainable and economically viable process to produce fuels and other vital chemicals. In the process chain the gasification is integrated and straw is converted via pyrolysis into a bioslurry consisting of a liquid, tar-rich phase and char. Afterwards, the slurry is gasified into a tar-free, low-methane syngas which is a basic reactant for the synthesis of biofuels. At the high temperatures over 1200 ◦C the ash constituents of the char in the bioslurry melt and flow down the inner wall as slag. The slag viscosity has to be in a certain range to form a protective layer at the reactor wall and to guarantee a continuous removing. For this reason, the composition of the molten ash at the reactor wall has to be well known. Due to several fractionation processes in the gasifier the composition of the slag at the reactor wall does not correspond directly with the slurry ash. Therefore, experiments were conducted to identify depletion and enrichment processes in the gasifier. Finally, the composition of the slag at the reactor wall is obtained and can be used for the adjustment of the viscosity.