This article is about the relationship between the properties of a refinery middle distillate feedstock and its reactivity toward hydrodenitrogenation (HON). The HDN experiments were conducted with 12 raw middle distillates over a sulfided CoMo/Al2O3 catalyst and with 13 prehydrotreated middle distillates over a sulfided NiMo/Al2O3 catalyst. Of the multitude of physicochemical properties, the feed nitrogen content, which usually reflects the heaviness of the feedstock, is a rough indicator of the HDN reactivity. This suggests a self-inhibition effect whose implications are discussed. (C) 2010 Elsevier B.V. All rights reserved.