Highly dispersed SrO in amorphous ZrO₂ or SiO₂, SrO–ZrO₂, and SrO–SiO₂ mixed oxides were synthesized by applying the solid–liquid interface reaction of Sr(OH)₂•8 H₂O in the solid phase with an equal number of moles of Zr(OC₃H₇)₄ or with twice the number of Si(OC₂H₅)₄ moles dissolved in 2-propanol. After heat treating at 723 K, the SrO–ZrO₂ catalyst exhibited a higher activity than the conventional solid base MgO catalyst for the base-catalyzed retroaldol reaction of diacetone alcohol. The catalytic activity of SrO–ZrO₂ was higher by a factor of 1.5 than that of MgO prepared by the thermal decomposition of hydrated magnesium oxalate (MgC₂O₄•2H₂O). By contrast, the highest activity of SrO–SiO₂ obtained by thermal activation at 673 K was much lower than that of MgO. The active SrO–ZrO₂ catalyst was composed of catalytically inactive species of SrZrO₃, and small amounts of Sr(OH)₂ and SrCO₃. The expected active species of SrO was not detected in powder X-ray diffraction analysis in any sample. The formation of SrO in small crystal size dispersed in amorphous ZrO₂ was strongly expected. Similar results were obtained in the SrO–SiO₂ sample. An active site formed on SrO–ZrO₂ surface is proposed.