The mechanism of electrical nucleation of aqueous sodium acetate trihydrate (SAT) solution is investigated. Direct observation of supercooled aqueous SAT solution by dynamic light scattering reveals the existence of large clusters with peak sizes of 0.01-0.8 mu m. A new method for preparing samples by using saturated aqueous NaCl solution as the floating medium of the SAT clusters is developed for this measurement. A strong dependence on the electrode material is found, suggesting that the electric field is not the driving force of nucleation. We conclude that dehydration of clusters by elution of Ag+ ions from the anode is the cause of electrical nucleation. This mechanism is supported by nucleation experiments using dehydration agents such as CaCl2 and molecular sieve.