This paper presents the results of a study to assess the slagging propensities of a suite of UK, Spanish and South African coals, ranging from lignites to anthracites. Laboratory deposits were collected on ceramic deposition probes at gas temperatures of similar to 1250degreesC, using an entrained flow reactor that simulates the time-temperature conditions experienced by pulverised coal particles in a large utility boiler. The degree of sintering and consolidation of the deposits would not have been predicted from bulk ash chemistry, indicating the importance of mineral matter distributions in the pulverised coal. Deposits with similar base to acid ratios and Fe2O3 contents displayed a range of slagging propensities on CCSEM analysis, consistent with the visual ranking. CCSEM analysis of the fly ashes collected from the combustion gases revealed a similar chemical composition to the coal ash and ash collected at the base of the EFR. CaO was observed to have readily assimilated into the aluminosilicate fly ash particles. On deposition, the CaO distribution largely remained unchanged. Fe2O3 was redistributed on forming a deposit possibly aided by CaO already dissolved in the aluminosilicates. The study provides an insight into the observations made by boiler operators burning coals with high CaO and Fe2O3 ashes.