Dissolution of kaolinite, halloysite and amorphous hydrous aluminosilicate in a 0.001 N HCl solution at 47°C has been investigated to study polymorphic transformations of these minerals in aqueous solutions. Chemical analyses of the solutions obtained show that the concentrations of Al, Si and H + change noticeably with time. However, the change of the concentrations are not described by any specific simple rate expressions. The loci of points on a log aAl3+ + 3 pH vs. log a H 4SiO 4 diagram indicate that the solution (1) has not reached any partial equilibrium when dissolving phase is kaolinite,(2) has reached equilibrium with kaolinite when dissolving phase is halloysite and (3) has reached equilibrium with halloysite when the dissolving phase is amorphous hydrous aluminosilicate. It is suggested that these polymorphic transformations proceed by dissolution of the original solid and precipitation of the more stable polymorph. The transformation may continue in a solution saturated with the more stable polymorph until the original solid disappears. This transformation mechanism explains the zonal distribution of allophane, halloysite and kaolinite in a weathering profile and occurrence of kaolinite-rich sediments.