The partition rate of acetylacetone between water and some water-immisible solvents was observed by the “in-liquid ejection” method at 25°C. The effect of base-catalyst, CH 2ClCOO − and CH 3COO −, in the aqueous phase and of several solvents as organic phases on the partition rate constants was examined. All of the results were fairly consistent with the simple relation which was derived based on the assumption that the rate-determining step of the reaction takes place in an aqueous phase, i.e. k p = A . k, where k p is the partition rate constant, A is the equilibrium factor composed of the apparent partition coefficient and the enol fraction of acetylacetone in each phase, and k is the keto-enol tautomerization rate constant in the aqueous phase. The dominant process determining the partition rate of acetylacetone was clarified to be the keto-enol tautomerization in the aqueous phase.