A mild solution method has been designed for the selective synthesis of orthorhombic and hexagonal CeOHCO(3), as well as cubic CeO(2) crystals in an ethanol/water mixed solvent. This study added a new example for selectively controlling different cerium compounds by manipulating the balance between kinetics and thermodynamics in a mixed solvent system. The competitive reactions taking place in the ethanol/water system, phase transition, and shape evolution were fully investigated: they were found to be strongly dependent on the composition of the reaction media. The influence of the ethanol content in the mixed solvent and that of the reaction time on the phase transition and shape of orthorhombic and hexagonal CeOHCO(3) crystals is discussed in detail. Metastable hexagonal CeOHCO(3) can be trapped, even at 80 degrees C, in the ethanol/water solvent mixture without the need for the high temperature adopted by previous hydrothermal approaches. The evolution process of orthorhombic and metastable hexagonal phases under mild solution conditions is discussed for the first time. Supersaturation will become faster and more evident when water is replaced by ethanol, because the inorganic salts have a lower solubility in ethanol than in water, and this will generally favor the formation of the kinetic phase, such as the hexagonal CeOHCO(3) phase reported in this paper. The optical properties of the products with different phases and composition were investigated.