The pressure derivative of the U-band peak position in KBr was determined to be +27×10−6 eV bar−1 and +30×10−6eV bar−1 for H− and D− impurities, respectively. These are equal, within the uncertainty of ±3×10−6 eV bar−1 for each coefficient. The shift of the peak of the U band with temperature is shown to arise almost entirely from thermal expansion.