The thiophene hydrodesulfurization (HDS) activity of Pt/ASA, Pt/SiO2, Ir/ASA and Ir/SiO2 catalysts at thiophene concentrations of 300 ppm has been studied at 573 K and 20 bar. Pt/ASA showed the highest HDS activity. Nitrogen tolerance was investigated by co-feeding ammonia in gas phase concentrations from 10 to 1000 ppm. The amorphous silica alumina (ASA) supported catalysts were similarly strongly inhibited by the presence of ammonia, with even 10 ppm of ammonia causing a significant drop in activity, despite widely different dispersions. The SiO2 supported catalysts were less severely affected by the presence of ammonia.