In view of the large scatter in the literature, we have re-evaluated the high-pressure behaviour and the bulk modulus of thorium dioxide, experimentally using high-pressure X-ray diffraction (XRD) in conjunction with synchrotron radiation. To compare with the experimental measurements, theoretical calculations were performed using the full-potential linear muffin-tin orbital method (FP-LMTO) together with the local density approximation (LDA) as well as the generalised gradient approximation (GGA). The experimental value for the bulk modulus is found to be 195.3 ± 2.0GPa, which compares well with the calculated value of 198GPa obtained by using the generalised gradient approximation to the exchange correlation potential. © 2004 Elsevier B.V. All rights reserved.