Fresh and pretreated (in H2S/H-2, H-2, or He) unsupported nickel-and cobalt-phosphorus compounds were characterized by XRD and EM/EDAX and tested in gas phase quinoline hydrodenitrogenation (643 K, 30 bar). During pretreatment or reaction, all compounds were transformed into Ni2P or Co2P. In addition, nickel supported on activated carbon, silica, silica/alumina, alumina, or NaY, to which phosphorus was added via three different methods, was tested in quinoline HDN, In most cases, the effect of phosphorus introduction was found to be strikingly beneficial, provided that the method of introduction was adjusted to the support : except with NaY, highly active molybdenum-free HDN catalysts could be prepared, The activity of the supported phosphorus promoted catalysts was ascribed to a new sulfur-free phase, consisting of highly dispersed Ni2P.