Abstract Accurate rovibrational analysis of the lower part of the Icosad 12 CH 4 spectrum in the 6539–6800 cm −1 region is reported. The analysis uses the WKLMC experimental line list previously obtained in Grenoble by Cavity Ring Down Spectroscopy and Differential Absorption Spectroscopy at T =80 K. Based on an ab initio potential energy surface, the full Hamiltonian of methane nuclear motion was reduced to an effective Hamiltonian using high-order Contact Transformations method with a subsequent empirical optimization of parameters. About 2445 experimental line positions were fitted with RMS standard deviations of 0.005 cm −1 and 1644 line intensities were modeled using effective dipole moment with the RMS deviation of 11.5%. Twenty five vibrational energy levels of the lower part of the Icosad were determined.