Experimental and simulation studies have shown that the gauche conformational degrees of freedom of long-chain amphiphile molecules assembled in a dense Langmuir monolayer play an important role in determining the structures of the several phases that the monolayer supports. Nevertheless, for simplicity the extant theoretical analyses of the Langmuir monolayer phase diagram ignore gauche molecular conformations, thereby treating the amphiphile molecules as rigid rods. We propose a description of the influence of the conformational degrees of freedom of a long-chain amphiphile molecule on the phase diagram of a Langmuir monolayer. Our analysis extends the Landau-type theory of the Langmuir monolayer diagram formulated by Kaganer and Loginov (Phys. Rev. E 1995, 51, 2237-2249). The gauche defects are represented by a secondary order parameter coupled to the Kaganer-Loginov primary order parameters (the tilt vector of the molecules and the density waves describing herringbone ordering along and normal to the line of centers between molecules). The effect of the gauche conformations is to modify the coefficients of the primary order parameters in the free energy expansion and thereby to change the location of the transition lines in the phase diagram. For transitions that are induced by a change in the surface pressure, the tilting transitions, we obtain a shift of the transition lines to lower surface pressure. For transitions that are induced by lowering the temperature, the crystallization transitions, we suggest, given some restrictions on the magnitude of the coefficients of the coupling terms, that the transition lines shift to lower temperature.