Supercooling phenomena of some hydrates such as Na_2S_2O_3・5H_2O is studied from the chemical potential difference of water molecules in molten and crystalline hydrate. These hydrates can be classified into two groups. One group has Δμ_<H_2O>>0 and nucleates with difficulty. The other group has Δμ_<H_2O>〜0 and nucleates very easily. Nucleating agents for these hydrates are discussed mainly on a point of view of the coordination structure of water molecules around Na ions in the crystalline hydrate. Interesting facts found for CaCl_2・ 6H_2O and the gas hydrate are given also. Essential requirements for the crystal structure of the nucleating agent are still open question.
