In this paper molybdenum oxide promoted graphite supported ruthenium catalysts highly active and selective for Fischer-Tropsch synthesis (FTS) are presented. The effect of Mo loading (0-5 wt%) of promoted graphite supported Ru (2 wt%) catalysts on the syngas conversion was studied at 523 K, H-2/CO = 2 and 3.5 bar. Mean diameters of Ru nanoparticles were all close to 3 nm independently of the molybdenum loading used. Microcalorimetric measurements during CO adsorption at 330 K reveal a clear interaction between Ru and Mo observing an important increase of CO adsorption heats for Mo/Ru ratios of <= 0.26. XPS analysis performed to 2Ru0.5Mo/G catalyst after in-situ reduction showed the presence of Mo-VI (MoO3) and Mo-IV (MoO2) species on the catalyst surface. It is argued that these Mo species could be located around the ruthenium nanoparticles acting as Lewis acids and therefore facilitating the CO dissociation. Mo promotion increased up to four times the activity with respect to unpromoted Ru catalyst, increased the selectivity to C5+ products and improved the olefin to paraffin ratio in the C-2-C-3 hydrocarbons range. These conclusions contribute to understand the Mo promotion effect in the FTS, because realistic deductions have been obtained using a proper inert support for studying the molybdenum promotion effect in ruthenium based catalysts, avoiding strong metal-support interaction effects.