The streaming potential of supersaturated solutions of K(3)[Fe(CN)(6)] and K(4)[Fe(CN)(6)] was determined to characterize crystallization under declining temperature conditions. The value of the streaming potential was related to the oxidation state of the iron atom in the complex, the solubility of the complex, and the starting temperature of crystallization. It was easily in by the the in the polarization of the electrode, the hydrolyzation of the complex, and the rate of the temperature decline. When the temperature dropped to the point where crystal nuclei appeared. the streaming potential reached a minimum Thereafter, as the number of crystal nuclei increased, some minicrystals grew, and the streaming potential presented a tendency to increase The higher the starting impel :mile of the complex saturated solution is, the higher temperature corresponding to the value of the streaming potential minimum is The temperature of the streaming potential minimum in the K(3)[Fe(CN)(6)] supersaturated solution is lower than that in the K(4)[Fe(CN)(6)] supersaturated solution