Sulfophthalein series dyes combine with organic bases to form addition products soluble in organic solvents and colorimetric determination of bases by the utilization of this nature is possible. Conditions for this determinination were examined with dl-methylephedrine hydrochloride and ephedrine hydrochloride and it was found that the dyes of halogen derivatives, such as bromothymol blue, bromocresol green, and bromocresol purple, and polar solvents such as ethylene dichloride and chloroform gave good results. Partition coefficients between the aqueous layer of the addition product and the organic solvent layer, under the conditions employed, were measured and a part of the mechanism of this determination method was clarified.Relationship between chemical structure and optical density was examined in several basic compounds, including official and new remedies. It was thereby revealed that pKb, kind of amines used, molecular weight, and the presence or absence of hydrophilic radical affected the optical density.