We have measured the Brillouin shift as a function of hydrostatic pressure in dense argon and nitrogen at different constant temperatures. We show that for argon there is a linear relation between the index of refraction and the reduced density. A comparison of the sound velocities determined ultrasonically and from our Brillouin data shows a negative velocity dispersion. This is in agreement with theoretical predictions and measurements of the temperature dependence of the sound velocity along the vapor-pressure equilibrium curve using Brillouin scattering. The fact that the negative velocity dispersion increase with pressure at constant temperature is smaller at lower temperatures is, however, puzzling and cannot be explained within the framework of the above-mentioned theoretical model.