The present study aims at utilising an inorganic industrial by-product, high calcium fly ash (HCFA), in an environmental field: oil spill clean up. Properties, such as fine particle size, floating ability, hydrophobic character and porosity, make this material attractive for such a use. In order to investigate the oil sorption behaviour of HCFA an oil spill has been simulated, by using artificial ocean water and three types of oil (heating oil (HO), light cycle oil (LCO) and Iranian light crude oil (ILCO)). Two HCFA samples, a Ca-rich one (AD) and a Si-rich one (M), have been examined, so as to investigate the role of HCFA composition in its behaviour. The addition of HCFA to an oil spill results in the formation of a semi-solid oil-HCFA phase, allowing the quite total removal of oil from the water surface. HCFA's oil sorption capacity in dry environment after 24 h is 0.7-0.9 g oil/g HCFA for AD and 0.5-0.6 g oil/g HCFA for M. HCFA's behaviour, when added to an oil spill, necessitates the amelioration of its floating ability and affinity for oil and to this direction its hydrothermal treatment in an aqueous solution of sodium oleate (SO) under several conditions (time, temperature, HCFA: SO mass ratio, SO solution concentration, solution: HCFA ratio) has been applied. The treatment of the calcareous HCFA (AD) at 25 degrees C at a mass ratio HCFA: SO = 1: 0.004 results in the formation of a cohesive semi-solid oil-HCFA phase, allowing the total removal of oil from the water surface, while the siliceous HCFA (M) requires a greater amount of SO (HCFA: SO = 1: 0.5). The oil sorption behaviour of both HCFA and SO-modified HCFA seems to be related to the initial CaO content. During the treatment with SO, CaO is converted to calcite and calcium oleate, which contributes to the improvement of HCFA's floating ability, due to its greater affinity for oil. (C) 2010 Elsevier Ltd. All rights reserved.