Static dielectric constant epsilon and refractive index n have been measured simultaneously over a large pressure range (approximate to 23 MPa). The measurements are carried out at temperatures in the supercritical region both for SF6 and CO2. From these static measurements, we determined the coefficients gamma(1), gamma(2),..., related to the linear, quadratic,..., density dependence. It is shown that the meaningful physical coefficients gamma(1), are related to the integrated absorption by means of the Kramers-Kronig relation. For SF6, the results are compared with the earlier dielectric and optical results. For CO2, the binary coefficients gamma(2) are in good agreement with the static and the absorption results. Furthermore, our results together with the accurate existing integrated absorption data demonstrate the temperature dependence of the gamma(2) component.