We have investigated the physical and chemical behaviour of pure-, mixed-, layered-, and thin-solid films of CO and CH4 using Fourier Transform Infrared Spectroscopy. Very little difference exists between the sublimation behaviour of the films. In contrast, a significant spectral feature was observed at similar to2136 cm(-1) only in the mixed CO:CH4 films, and was replicated by ab initio calculations on the gas-phase CO-CH4 complex. The complex is due to a 'pseudo-double pi-hydrogen bond', with binding energy of around 14 kJ mol(-1). To the best of our knowledge this is the first report of this complex. (C) 2003 Elsevier B.V. All rights reserved.