The scale of the thermodynamic acidity of HCl solutions in DMF has bee
n measured at 25 and 39.5 degrees C up to 15 mol L(-1) acid concentrat
ion by the indicator method. It is shown that in the HCl-DMF system th
e ionization of the nitroaniline-derived indicators occurs by the ion
pair mechanism. Within the temperature range studied acidity is indepe
ndent of temperature. Indicator; of the nitroaniline series are used.