The steam reforming (SR) of ethanol/phenol mixture (168 gTOT/N m3, ethanol:phenol 2:1 mol, GHSV = 54,000 h− 1), assumed as a model for tar mixtures, has been studied over a 5% Ni/Al2O3 catalyst (155 m2/g), in the presence and in the absence of 210 ppm tetrahydrothiophene (THT) as a sulphur containing contaminant. The sulphidation of the catalyst by THT has been studied by IR spectroscopy. Infrared spectra of CO adsorbed at low temperature over the oxidized, the reduced and the sulphurized catalyst have also been recorded. The catalyst acts as a bifunctional one, with the behaviour attributed to the uncovered support (alumina modified by nickel ions) at 773 K (dehydration of ethanol to ethylene, dehydrogenation to acetaldehyde and alkylation of phenol with ethanol) that fully disappears at 973 K when steam reforming occurs very selectively. By lowering back the reaction temperature, the support behaviour reappears. THT poisons selectively the Ni component, thus causing the appearance of the support behaviour also at 973 K. IR experiments show that THT deposes sulphur at the catalyst surface with the production of gas-phase 1,3-butadiene, thus converting the catalyst into a “sulphided” SR-inactive state. The steam reforming activity of the poisoned catalyst progressively reappears upon feeding back S-free feed at 973 K. IR study suggests that steam “cleans” the catalyst surface by sulphur, generating a “disordered” surface with dispersed Ni2 + and Ni0 species, that could slowly re-approach the initial active state.