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Cement pastes containing 0％, 15％, 25％ and 35％ fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 μm and then the powders were immersed in alkali solutions. Adsorption characteristics of K+ and Na+ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K+ and Na+ ions in the pastes were also investigated. Results showed that the contents of K+ and Na+ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K+ or Na+ that was essentially independent of alkali concentration. Adsorption mechanism of K+ and Na+ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K+ and Na+ ions in the pastes is reversible. The inherent K+ and Na+ ions in the pastes entered rapidly into the de-ionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K+ and Na+ ions which were originally “bound” by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.