The phase diagrams of methane-benzene and methane-1-methylnaphthalene binary mixtures have been measured using an infrared absorption method. The measurements have been performed at 323 and 373 K for the first mixture and at 295, 323, 348, 373 and 398 K for the second. The special high pressure cell (200 MPa, 473 K), which has been built in order to study mixtures containing heavy components, is presented. With the mathematical treatment presently used the measured concentrations are insensitive to the baselines. Therefore, it is possible to measure the concentration of each species in the gas and in the liquid phase in all parts of the phase diagram : bubble curve, dew curve and even the critical region. Our results are compared to those of different authors, although data on methane-heavy hydrocarbons are very scarce. Data on the methane 1-methylnaphthalene mixture are used to estimate new interaction parameters for the Peng-Robinson equation of state at the different temperatures. Finally, we show the importance of obtaining reliable data in all parts of the phase diagram to determine a K-ij value, which allows the best fit of the whole phase envelope.