The transient sulfur adsorption and reaction on partially sulfided and oxyregenerated Co-Mo-Al catalysts in the hydrodesulfurization of heavy atmospheric gas oil (HAGO) was studied in batch and trickle-bed reactors. All experiments were carried out at 360 degrees C and 5.5 MPa. The contribution of sulfur adsorption to sulfur removal was significant in cases of 1000-1800 ppm sulfur in the feedstock. The sulfur level in hydrotreated HAGO was lowered by 50-350 ppm due to sulfur adsorption on the catalyst for periods of 20-50 h. The principle of regeneration and adsorption in a trickle bed has been proven. A model that describes the transient adsorption and reaction is proposed.