Alkali treatments (elution and activation) are known to promising processes for recycling sewage sludge ashes. In this study, we examined the dissolution and solidification property of the major elements of sewage sludge ashes, especially focusing on P, Al, and Si. Under alkali elution conditions (water/solid ratio is ＜ 1) , the elution of P, which was derived from the dissolution of aluminum phosphate and iron phosphate, and that of Al, which was derived from the dissolution of aluminum phosphate, were two dominant reactions. Under alkali activation conditions (water/solid ratio is ca. 10), the elution of Si and the solidification of Al (i.e., fixation as alminosilicate) were observed in addition to the elution of P and Al. Most of the eluted P was not fixed by alkali activation. These alkali-related reactions were promoted by higher concentration of aluminum phosphate, which had correlation to the aluminum contents in the ash samples. Specific surface area of the ash samples also had positive effect on the alkali-related reactions. Considering that sewage ashes collected in Japan generally contain higher concentration of Al than those collected in Europe, alkali elution was more suitable option for Japan.