Chlorophyll-a (chl-a) was found to easily change to pheophytin-a during its adsorption on hexagonal mesoporous silica (HMS) since the centered Mg ions of chl-a easily react with silanol groups as Bronsted acid sites (pheophytinization). This pheophytinization process Could be Suppressed by adding small amounts of Mg species into the frameworks of HMS. The highly dispersed Mg species work as adsorption sites for chl-a and/or the porphyrin ring of pheophytin-a, resulting in the formation of an original chl-a like complex species.