Insoluble surfactant monolayers at the air/water interface undergo a phase transition from a high-temperature homogeneous state to a low-temperature demixed state, where dilute and dense phases coexist. Alternatively, the transition from a dilute phase to a dense one may be induced by compressing the monolayer at constant temperature. We consider the case where the insoluble surfactant monolayer interacts with a semidilute polymer solution solubilized in the water subphase. The phase diagrams of the mixed surfactant/polymer system are investigated within the framework of mean field theory. The polymer enhances the fluctuations of the monolayer and induces an upward shift of the critical temperature. The critical concentration is increased if the monomers are mure attracted (or at least less repelled) by the surfactant molecules than by the bare water/air interface. In the case where the monomers are repelled by the ban interface but attracted by the surfactant molecules (or vice versa), the phase diagram may have a triple point. The location of the polymer special transition line appears to have a big effect On the phase diagram of the surfactant monolayer.