Using numerical self-consistent-field (SCF) calculations and analytical theory, we investigate the behavior of polymer brushes immersed in a mixture of two immiscible solvents. When only a small amount of one of the solvents is present in the mixture, the fluids can phase separate within the brush. We find that the grafted polymers saturate the solvent-solvent interface, and we analyze this effect as a function of solvent composition and grafting density. Modifying the SCF method, we also examine the behavior of a weak polyacid brush immersed in a mixture of oil and water. Now, the polymer segments in water can become ionized; the segments in oil remain neutral. The results demonstrate that the brush height is readily controlled by varying the amount of water in the solution. The findings provide guidelines for tailoring the adhesion or lubrication between polymer-coated surfaces or the steric stabilization of colloidal particles.