The growth-starting supercooling Delta T-G a degree of supercooling at which a crystal starts to grow when the solution is continuously cooled, was measured in supersaturated aqueous solutions in the presence of citric acid as an impurity(1)). It was measured both for the (100) and (111) faces of sodium chloride crystals. From the measured data of Delta T-G, the Langmuir constants of the (100) and (111) faces were determined at pH=4.5 to be 6.16 +/- 0.88 L/mol and 12.5 +/- 2.0 L/mol, respectively. This Delta T-G method is a convenient method for the determination of adsorption isotherms of an impurity onto specific crystallographic faces of a crystal.