Pressure-volume-temperature measurements were carried out on two van der Waals liquids, 1,1(')-bis(p-methoxyphenyl)cyclohexane (BMPC) and 1,1(')-di(4-methoxy-5methylphenyl)cyclohexane (BMMPC). In combination with dielectric spectroscopy results, the relative contribution of temperature and density to the structural relaxation times were quantified. We find that the ratio of the isobaric expansion coefficient [-rho(-1) (partial derivativerho/partial derivativeT)(P), where rho is mass density and T is temperature, evaluated at P=0.1 MPa] to the coefficient of isochronal expansivity [-rho(-1) (partial derivativerho/partial derivativeT)(tau), evaluated at tau=1 s] equals 0.58 and 0.72 for BMPC and BMMPC, respectively. This indicates that density exerts more influence on the structural relaxation times than does thermal energy. Corroborating this finding, the ratio of the isochoric activation energy to the activation energy at constant pressure is determined to be ca. 0.4 at ambient pressure for both glass formers. The prevalence of density over thermal energy is contrary to prevailing ideas concerning the dynamics of supercooled liquids, and must be taken into account in developing models of the glass transition. (C) 2003 American Institute of Physics.