(1) The distribution of diethyl tartrate between ether and water containing boric acid, borax or potassium metaborate (KBO2), has been measured. (2) The results of the experiment on the system of ether–water have been found to follow simply the distribution law of Nernst. (3) The distribution ratio in the system of ether-aqueous borate solution has been found to increase with the increasing electrolytic dissociation of borate, and this relation has been explained theoretically. The order of the magnitude of the distribution ratio to ether is water<borax<potassium metaborate. (4) The equilibrium of boro-diol complex m the borate phase has been studied, and it has been shown that in the borate phase two complex compounds, i.e. those of mono-diol and di-diol types, must coexist, and the equilibrium constant (between ions) has been estimated. (5) By the study of the temperature coefficient, the reaction of complex formation has been found to be exothermic and the heat of reaction has been estimated. (6) The distribution of diethyl tartrate in the system of ether–water has been found indifferent to the addition of free boric acid, and this fact has been explained to be due to the insignificant dissociation of boric acid.