A very selective isomerisation catalyst, i.e. molybdenum oxycarbohydride, was synthesised by isothermal activation of MoO3 by an H-2/n-C4H10 mixture at 350 degreesC. The compound was characterised by several techniques, including in situ XRD, temperature programmed oxidation (TPO) and temperature programmed desorption (TPD). In situ TPO and in situ TPD evidenced the presence of carbon and hydrogen inside the material, and enabled an estimation of the stoichiometry of the catalyst. A molybdenum oxyhydride, isostructural to the oxycarbohydride, was also synthesised by isothermal activation of MoO3 with pure hydrogen at 350 degreesC. This compound was then characterised before and after exposure to the H-2/n-C4H10 mixture. In situ TPO proved that in the presence of a carbon source, the oxyhydride rapidly picked-up some carbon to form the previous oxycarbohydride, thus, explaining why similar catalytic behaviour was reported for both compounds. The MoO2 suboxide was found to be a poor isomerisation catalyst, because of its inability to be a precursor of the oxycarbohydride phase.