Gas phase hydrogenation of toluene, diluted in inert n-hexane in order to minimise the thermal effects was investigated in a fu;ed bed reactor at 110 degreesC over series of bifunctional catalysts in which Pt or Pd was deposited on alumina or on an acidic HFAU zeolite. Practically no deactivation was observed when the metals were deposited on alumina, even in mixture with the HFAU zeolite, whereas there was a rapid initial deactivation when Pd and especially Pt were deposited on the acidic zeolite. However, even in this latter case, the activity of the fresh catalysts can be obtained with a good accuracy thanks to the use of a multiple loop value. For given metal and support, a linear correlation was found between the hydrogenating activity and the number of accessible metal atoms. However, Pt was found to be 20-60 times more active than Pd, both metals being more active on HFAU zeolite than on alumina. This acidity effect could be explained by the hydrogenation of aromatic molecules adsorbed on acidic sites by hydrogen spilled over from the metal surface. The initial deactivation of Pt- and Pd-HFAU zeolite catalysts was due to the rapid formation of 'coke' molecules inside the zeolite micropores. 'Coke' was mainly constituted by C-14 and C-21 products resulting from the acid alkylation of toluene molecules by the olefinic and dienic intermediates of their hydrogenation. While the composition of 'coke' as well as its deactivating effect did not depend on the metal (Pt or Pd), the rate of 'coke' formation at isoconversion of toluene was found to be lower on Pd than on Pt zeolite catalysts, which explains their slower deactivation.