When magnesium oxalate hydrates were precipitated by mixing ethanolic solutions of the metal nitrates and oxalic acid with addition of water, particular attention was paid to the age of the starting solutions, which had drastically influenced the precipitation behavior. For instance, the aging time of several days exponentially delayed the occurrence of the clouds in the solution, in other words, elongated the incubation period of the precipitation. Although the structural features in the aged solution have not yet been clarified by NMR, we believe that a visibly transparent solution is not always homogeneous, even if it looks stable for a long period.