Abstract: Diphenyl amine has been found to be an excellent internal indicator for diazo- tization method of determination of aromatic amines, but the mechanism of colora- tion is still to be studied. From the results obtained under various experimental conditions, it is con- cluded that various aromatic amino-drugs, including sulfa-drugs, local anesthetics, systemic anti-infectives, etc. can be determined by using this indicator, the results are in good agreement with those obtained by the classical method. A1 %concentration of diphenylamine is suitable for indicating purpose, the color produced is stable even after 24 hours, and the indicator correction is found to be 0.05 ml M/10 NaNO₂ solution. A high speed of titration gives high results, but the effect of temperature is rather small, the titration can be carried out at a temperature as high as 27℃ with no appreciable error.