Hydrotreating is the response to the necessity of a cleaner feed for downstream processes and reduced pollution. Hydrotreating catalysts are vital in this process; hence catalyst deactivation is a key issue. The principal objective of this research was the experimental study of hydrotreating catalyst deactivation due to arsenic compounds. The hydrotreating of light gas oil, in the presence and absence of an arsenic compound over a commercial NiMoS catalyst, was investigated in a trickle bed reactor (temperature 315-360˚C, space velocity = 1-3 h-1, pressure = 3MPa). Kinetics of first order for nitrogen and sulphur were found and activation energies values of 32 kj/mol and 76 kj/mol respectively. Studies of activity changes, suggested that arsenic mainly affects the conversion of sulfur compounds; which might indicate that arsenic prefers mainly the S edge of the catalysts. Activation energy values decreased after arsenic introduction, which may suggest pore plugging of the catalyst.