Several series of commercial alumina supported nickel-molybdenum and cobalt-molybdenum hydrotreating catalysts were exposed to differing degrees of reaction lengths and severities using a variety of naphtha and gas-oil feeds. Fresh and used catalysts were characterized by gas-oil desulfurization and denitrogenation activity, chemical analysis of deposits, porosimetry, and high-resolution transmission electron microscopy; in addition, deposits on the catalysts were characterized by carbon-13 nuclear magnetic spectroscopy. Deactivation was primarily by suppression of active sites by carbon containing deposits on the support. Changes in MoS2 structure, as observed by HRTEM, do not appear to contribute to activity changes. Two types of carbon deposits were observed : processing with gas oils leads to deposition of a compact carbon, resulting in substantial loss in nitrogen removal activity but only slight loss in sulfur removal activity; at higher loadings this dense carbon deposit affects both sulfur and nitrogen removal activities, but nitrogen removal is reduced to a greater extent. Processing with naphthas results in deposition of less dense carbon, which reduces nitrogen and sulfur removal by the same amount, the extent of which is proportional to carbon loading. Comparison of different catalysts deactivated in the same commercial environment permits insights into those factors important to preventing activity loss.