We have investigated the preferential solvation effect in poly(styrene) brushes immersed in a binary mixture. The small angle neutron scattering technique has been used for determining the chemical composition of the mixture trapped into the polymer interface. When both of the solvents are good for the polymer, no effect has been noticed. However, for a mixture of acetone and cyclohexane (both poor solvents for the poly(styrene)), we have observed that this mixture can behave as a good solvent-cosolvency effect. The thickness of the interface can be twice that in either pure acetone or pure cyclohexane. Moreover, the chemical composition of the mixture trapped into the brush is different from that of the bulk : it is enriched in the component which tends to achieve the best composition (for which the thickness is maximum). Finally, for a mixture of a good and a poor solvent, we recover the same nonlinear behavior as we have described previously (ref 3). This is associated with a strong preferential solvation of the good solvent.