Compressibility of MgCO3 is studied with infrared absorbance in the 100-1800 cm(-1) wave number range. Large values of the Gruneisen parameters for the translational modes, related to the optically active mode in the B1 structure, indicate that the pressure induced changes in MgO6 octahedra account for nearly all of the compressibility, while the CO32- ions are essentially rigid. The application of the asymmetric C-O stretching vibration of the carbonate group as a pressure gauge for infrared transmission studies in a diamond anvil cell is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.