The nonlinear refractive index has drawn considerable attention these past few years, because it may significantly perturbate long-range propagation of lightwaves in optical fibers. Two main origins to this nonlinearity have been identified: nonresonant electronic nonlinearity and electrostriction. This paper shows that the electrostrictive contribution to the nonlinear refractive index n, may indeed be straightfowardly calculated from Brillouin gain measurements. The Brillouin linear gain g, is dependent on material properties and in particular on the electrostrictive coefficient.