Recently, our group synthesized perovskite-type oxide ABO3 (A = Sr, Ba; B = Ti, Zr) with high crystallinity simply by leaving a powder mixture of A(OH)2•nH2O and BO2 hydrous gel at room temperature to 323 K for 3–10 days. In this study, perovskite solid solution (Ba, Sr)TiO3 and Ba(Zr, Ti)O3 were synthesized to investigate the reaction mechanism in this synthesis method in detail. The perovskite solid solution (Ba, Sr)TiO3 was obtained by using barium hydroxide, strontium hydroxide and TiO2 hydrous gel as the staring materials. On the other hand, in the case of using barium hydroxide and the mixture of TiO2 hydrous gel and ZrO2 hydrous gel as the starting materials, a mixed phase of BaTiO3 and BaZrO3 was obtained. B-site-substituted perovskite solid solution Ba(Zr0.45Ti0.55)O3 was obtained by using barium hydroxide and (Zr0.45Ti0.55)O2 hydrous gel as the starting materials. From the above, it was clarified that in this synthetic method, the Sr and Ba ions diffuse into the water part of hydrous gel and then slowly react with Ti(OH)4 to form a perovskite lattice.