Coal is generally accepted to be a heterogeneous resource where coal properties can vary extensively between geographical sites or within a mine. However, detail coal characteristics are essential to predict gasification performance. Mineral matter transformation and slag formation are specific properties of a coal source that provide more information on the suitability for combustion or gasification purposes. Therefore, the chemistry and mineral interaction have to be understood in order to determine the suitability for fixed bed gasification purposes with regards to mineral matter transformations and slagging properties. The principle aim of this paper is to understand the chemistry and interpret mineral matter transformation during Sasol-Lurgi fixed bed dry bottom (S-L FBDB) gasification by means of high temperature X-ray diffraction (HT-XRD), in combination with FactSage modelling. Conventional ash fusion temperature (AFT) analyses are currently used to predict stagging properties of coal sources. Normal AFT analyses give an average flow property and do not indicate exactly at what temperature the first melt/slag is occurring. Operating experience indicates that even when the gasifiers are operated at temperatures above the flow temperature as given by AFT analysis, a low percentage of slag is formed. The HT-XRD findings were further supported with FactSage thermochemical modelling of the gasifier, and indicated that feldspar formation (including anorthite) correlated with slag formation at temperatures around 1000 degrees C. It can be concluded that HT-XRD and FactSage modelling supply insight into specific mineral reactions and slag formation. Although the amount of melt was fairly low at 1000 degrees C, a percentage of melt is definitely present, which at this temperature is not reflected by AFT analyses. The specific value for Sasol in using FactSage, in combination with HT-XRD is that it can be used to analyse equilibrium conditions for reactions occurring between inorganic and organic materials together, as well as provide insight into mineral transformation and slag formation. This can improve the interpretation of flow properties of the reacted mineral matter in coal and assist in identifying and quantifying slag formation in gasifier operation at temperatures not reflected by normal AFT analyses. (c) 2006 Elsevier Ltd. All rights reserved.