Gasification slag is a by-product of the coal gasification process. To achieve "zero emissions" from coal gasification technology, the environmentally safe utilization of slag by-products from gasifiers must be addressed and developed. In this study, work has been carried out to characterize residual carbon in slag together the morphological study on vitreous material present in slag. The compositions of particles in coal, char and slag were determined by using SEM/EDX. An elemental analyzer (Flash EA 1112) was adopted to analyze CHNS on bulk coal, char and slag. The size distribution of carbon in slag was obtained by analyzing slag sample blocks using an image analysis program developed in this study. It was found that the coarse and fine slags both have a relatively high content of unburnt carbon which hinders their utilization as additives in cement and concrete. There was no evidence that coarse slag is the final destination for coarse char particles since large unburnt chars were found in both the coarse slag and the fine slag. The morphology of the residual carbon in the coarse slag is similar to that found in the fine slag, and is also comparable to partially combusted char produced in a drop tube furnace (DTF) operating at 1300 degrees C, 600 ms and 2% oxygen. It is also found that the vitreous particles in fine slag tends to be below 150 mu m and the majority of residual carbon (75.6 wt%) in fine slag tends to be greater than 150 mu m, which implies that, a simple screening operation might make the slag usable in concrete industries. 2006 Elsevier Ltd. All rights reserved.