The phase diagrams of methane-hexane and methane-benzene mixtures have been determined at 323 K and 373 K using an in-situ infrared absorption method. The mathematical treatment of the absorption spectra allowing the determination of the molar density of each species in each phase is described with a special emphasis on the problem of baseline drift. The results obtained for both the bubble and the dew curves are in good agreement with those previously published by other authors using different experimental methods. Moreover the present in-situ measurements allowed us to make an accurate investigation of the critical region. These new data are compared to the phase diagrams computed either with the well-known Peng-Robinson equation of state (Peng and Robinson, 1976) associated with the classical Van der Waals mixing rules or with the predictive method based on group contributions developed by Abdoul and Peneloux (1991).