The dissolution of air-cooling and water-cooling blast furnace slags, a silico-manganese slag and a convertor slag in distilled water was examined. The solution pH was maintained constant at 4, 5, 6 or 7 during the dissolution experiments by the addition of hydrochloric acid with an autotitration system in order to study the effect of solution pH on the solubilities of the slags. The amounts of dissolved Si, Ca, and Mg increased with decreasing solution pH. The solubility of the convertor slag was highest among the tested slags in terms of the percentage of dissolved Si to total Si in the slag when solution pH was from 5 to 7, but was lower than those of blast furnace slags at pH 4. Ca/Si and Mg/Si mole ratios in the solution decreased with extraction time except for the convertor slag, suggesting preferential dissolution of Ca and Mg. The slags were also dissolved in calcium chloride solution to study the effect of Ca concentration on Si dissolution from the slags. The amount of dissolved Si decreased with increasing Ca concentration in the solution. The increase in solution pH caused by Ca and Mg dissolution from the slags was suppressed by high Ca concentration in the solution. The dissolution process of the slags in water was considered as follows : i) Ca and Mg are dissolved by ion exchange reaction with hydrogen ion in water (first step). ii) Si-O-Si and/or Al-O-Si chemical bonds are cleaved by hydrolysis (second step). The second step was not necessary for Si dissolution from the convertor slag containing β-larnite as a main mineral.