Abstract: This paper describes a method for the determination of armillarisin A. Armillarisin A is a new compound of the coumarin type. In acidic and alkaline solutions Armillarisin A changes between lactone and coumarinic acid forms. At pH<4.5 it exists entirely in lactone form and the absorbance does not change with pH. At pH 4.5～8.0 it exists in both forms and the absorbance changes considerably when pH changes slightly. At pH 8.0～10.0 it exists entirely in coumarinic acid form and the absorbance does not change with pH. At pH 11.0 the coumarinic acid form is unstable and the absorbance is unsteady. Armillarisin A is stable in a buffer solution of pH 8.0～9.0 for 2～3 hours. Based on this study a borax-HCl buffer solution of pH 8.0～9.0 was chosen as solvent. The absorbance was measured at 420 nm against a reagent blank. The method is simple and gives steady and reproducible results. The average relative deviation of six determinations is less than 0.2% and that of fourteen batchs of samples is less than 0.4%.