An investigation was made to study the influence of low-cost filler material such as non-coking coals and refuse-derived fuel (RDF) on coke quality. Interfaces between textural components within the cokes were successfully characterised and the derived interface quality index showed some cokes contained more 'good' quality interfaces than others. The addition of filler coals and RDF to the coking coal increased the proportion of 'poor' interfaces'. A good correlation between coke strength, derived from a small drum test, and interface quality index was observed. During heat treatment of cokes at 1600 degrees C both metallic and non-metallic micro-constituents were found to undergo some transformation as revealed by the SEM surface morphology examination. Although heat treatment caused some fractures to enlarge and others to emerge, its effect on the quality of the coke was not significant. Based on the results from the samples studied, there were some indications of the potential use of RDF material in the production of coke as there were minimal adverse effects on the quality of coke produced. (c) 2007 Elsevier Ltd. All rights reserved.