The temperature and pressure dependences of the nuclear quadrupole coupling frequency of Na²³ in NaBrO₃ have been measured, The quadrupole frequency is found to range from 886 kHz. at 4.1 °K to 849 kHz. at 300 °K at atmospheric pressure. The pressure dependences were determined at 196, 273 and 298 °K. From the pressure dependences and the compressibility the frequency was found to vary with the volume about as V^-2.5 at all three temperatures. The rate of change in frequency with temperature at a constant volume, 1/<i>v</i>(∂<i>v</i>/∂T)_V, was found to have a positive value and to decrease in magnitude with increasing temperature above 196 °K. / Ph. D.