Abstract The hyperfine coupling constant, A 0, of peroxylamine disulfonate in aqueous solutions depends upon the temperature and the concentration of added K 2CO 3buffer. Near room temperature, A 0varies from 13.036 to 13.201 G for K 2CO 3concentrations varying from zero to near saturation. For samples prepared in 50.0 m MK 2CO 3, the results are independent of Fremy's salt concentration in the range 0.005–2.8 m Mas follows: A 0( T) = 12.978 + 0.00311 T, where the hyperfine coupling constant at temperature T, A 0( T), is given in gauss when the temperature is given in °C. This temperature dependence is an order of magnitude larger and of opposite sign than that found for doxylstearic acid esters and is proposed as the basis of an internal thermometer for EPR spectroscopy. The variation of A 0with solvent polarity is found to be a factor of about 27–30 less than for the neutral radicals di- tert-butyl nitroxide and doxylstearic acid esters. It is shown that microwave heating of aqueous samples at high microwave powers can be monitored by measuring A 0while conventional thermometry can lead to significant errors even for a thermocouple immersed within the sample just above the microwave cavity. The value of A 0(25°C) = 13.056 ± 0.002 G is significantly different than previously used standards; therefore, some previous data may require recalibration. Correction procedures for nonlinearities in the field sweep are presented.