Dissolution properties of three kinds of fly ash including a Pressurized Fluidized Bed Combustion (PFBC) ash from a novel type of fluidized bed combustion boiler have been studied by leaching in different concentrations of caustic soda solution up to 15 N at 25 degrees C and 80 degrees C, stirring for 1 h with liquor/solid ratio = 10. Liquors sampled by a centrifugal separator were chemically analyzed by ICP technique. A blast furnace slag was also studied for reference. Results showed that Ca2+ dissolved relatively large amount in genuine water, but no marked dissolution was detected with increasing NaOH concentrations of the leaching solution. No enhanced dissolution of Al3+ was detected for ordinary fly ashes collected from pulverized coal combustion boilers even with increasing concentrations of solution at 25 degrees C, whereas enhanced dissolution of Al3+ was observed at 80 degrees C, when 5 N was reached and more. On the contrary, appreciable increasing was noted on the dissolution of Si4+ with increasing NaOH concentrations even at 25 degrees C for the ordinary fly ashes and the Si4+ dissolution became enhanced at 80 degrees C, specifically in 5 N and more.