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The ESCA spectrum of the 1s line from carbon in methane has been recorded and calibrated against the 2p line of argon using a new high-resolution multipurpose ESCA instrument with monochromatised Al K alpha radiation. For the first time vibrational structure in a core-electron ESCA line is resolved. The energies, relative intensities and inherent widths of the resolved components of the core line have been accurately determined. Inclusion of the Doppler broadening effect is shown to be essential for the natural linewidth determinations. Energies and relative intensities have also been calculated using a contracted configuration interaction scheme (CCI) and a complete active space self-consistent field (CASSCF) program. Although the calculations reached well beyond the Hartree-Fock level, incorporating a large part of the correlation energy, the shape and position of the potential curves were still found to be …