The mechanism of liquid-phase catalytic hydrodenitrogenation at 3.1 MPa on silica-supported molybdenum phosphide, MOP/SiO2, and tungsten phosphide, WP/SiO2, was studied using a series of pentylamines of different structures. The reactivity pattern suggested that removal of nitrogen occurred primarily by an E2 elimination mechanism involving acidic and base sites on the catalyst surfaces in a push-pull process. Infrared spectroscopy and temperature-programmed reaction studies of ethylamine indicated that alkyl ammonium species formed on Bronsted acid sites were intermediates in the reaction. Similar results were obtained with a reference MOS2/SiO2 sample tested at the same conditions. This suggested that sulfur was probably present on the active surface and assisted in the removal of sulfur.