When silicate content is determined by the colorimetric method, the silicate content in the mineral waters and sodium silcate solution decreases in addition of ammonium hydroxide and sodium carbonate respectively, and moreover after addition of aluminium ion in the samples, when aluminium hydroxide is precipitated from ammonium hydroxide solution, total silicates in various forms are coprecipitated with it. The silicate content in the mineral waters that are kept in concentrations of 0.1～0.4 normality of sodium hydroxide showed special variations with times.