Various fly ash samples with different unburnt carbon contents were collected, characterised and tested for adsorption of basic dyes, Methylene Blue and Crystal Violet, in aqueous solution. It was found that unburnt carbon plays a major role in dye adsorption. The mineral matter of fly ash has little adsorption capacity and most of the adsorption capacity of fly ash can be attributed to the unburnt carbon. The fly ash with higher unburnt carbon content will have higher adsorption capacity. For the carbon-free fly ash, adsorption capacities for Methylene Blue and Crystal Violet are only 2 x 10(-6) mol g(-1) and 1.0 x 10(-6) mol g(-1), respectively, while the adsorption capacities for Methylene Blue and Crystal Violet on carbon-enriched fly ash are 1.2 x 10(-4) mol g(-1) and 1.0 x 10(-4) mol g(-1), respectively. A two-site Langmuir adsorption model best describes the adsorption isotherm. (c) 2005 Society of Chemical Industry.