Abstract: A comparison was made between the efficiencies of polyoxyethylene castor oil C-125 and Tween-80 in the solubilization of calciferol. Experimental results showed that there existed a linear relationship between the apparent solubility of calciferol and concentration of the two solubilizers. The solubilization efficiency of polyoxyethylene castor oil C-125 was approximately four times that of Tween-80. Ultraviolet absorption spectrum studies suggested that the solubilized molecules were located nearer to the polyoxyethylene group within the polyoxyethylene castor oil C-125 micelles. This might play an important role in making the differences in the solubilization efficiency.The stabilities of calciferol solubilized in the two solubilization solutions were determined by the initial average rate method, t_0.9^20° was 1294.9h for the polyoxyethylene castor oil C-125 solution and 153.5h for the Tween-80 solution. It was also found that the energies of activation were 14.2 and 9.3 kcal/mol, respectively. It appears that this might be the cause of the differences in the stability of solubilized calciferol.