The sensitivity of the phenolphthalein spectrophotometric method for the determination of the purity of β-cyclodextrin (β-CD) was studied under different conditions. The alkalinity of the phenolphthalein colorimetric solution and the concentration of phenolphthalein in the colorimetric solution affected the sensitivity of the spectrophotometric readings and the linear range of the calibration curve. The optimum conditions for a basic working solution at pH 10.53 were that 5 ml of phenolphthalein working solution and 5 ml of sodium borate working solution were made up to a total 50 ml of phenolphthalein colorimetric solution, and the optimum conditions for a basic working solution at pH 11.23 were that 5 ml of phenolphthalein working solution and 3 ml of sodium borate working solution were made up to 50 ml total solution. Because of the instability of the absorbance of phenolphthalein solutions, it is suggested that at over 98% purity, β-CD should be determined by other more accurate methods.