The ability of simple inorganic reactions such as NaOH + CuCl2, NaOH + AgNO3, and CuCl2 + K-3[Fe(CN)(6)] to form various complex precipitation patterns has recently been discovered. When the first reagent penetrates into a hydrogel by diffusion and reacts with the second reagent homogenized in the gel, a precipitate is built up that has two kinds of bordering surfaces with different properties. Our experimental results suggest that one of these surfaces is permeable, while the other one is an ion-selective membrane that restrains the diffusion of the first reagent's reacting ion, preventing the reactions from proceeding ahead of the membrane. The presence of an intermediary compound, the precursor of the precipitate, is observed in the front of the permeable surface.