Recycling of coal fly ash is an emergency issue to be addressed by the international community. The coal fly ash is mainly composed of anhydrous oxides of Si and/or Al, and contains much Si-O-Si and Si-O-Al bondings in its structure. We treated the coal fly ash with up to 0.5 M LiOH, NaOH and KOH aqueous solutions at up to 573 K within 3 h to generate hydroxyl groups on the surface of the ash. Cation exchange capacity values of the products measured at equilibrium pH of 10 tended to increase with increasing alkali concentration and temperature of the treatment. Weight losses of the products between 473 K and 773 K measured by thermal analyses were more than 1 %, while that of the untreated coal fly ash was almost zero. These observations indicated that hydroxyl groups, presumably Si-OH, were formed by the sub-critical water treatments. In case of the treatments with NaOH, zeolite species, Na-P1 and analcime, were formed at higher NaOH concentrations and temperatures. When LiOH and KOH were used, no crystals were formed, however by scanning electron micrograph, amorphous materials were observed on the surface of the coal fly ash particles. The amorphous materials were suggested to be formed with dissolution of Si and Al from the coal fly ash, followed by precipitation composed of Si, Al and alkali metal cations on the surface of the coal fly ash particles. In conclusion, the treatment with sub-critical water can generate hydroxyl groups on the surface of coal fly ash, and the treated coal fly ash is possibly available as adsorbents with cation exchange ability.