The effect of mommorillonite and kaolinite clays on fine coal flotation was investigated. A novel method based on zeta potential distribution measurement to study interactions of fine clay with coal was described. The zeta potential distribution measurement was performed with coal and clay suspensions, individually and collectively. The results were used to explain the observed depression of coal flotation by montmorillonite but not kaolinite clay addition. The settling tests confirmed that slime coating of montmorillonite clay on coal is detrimental to coal flotation. (C) 2003 Elsevier Science B.V. All rights reserved.