A series of co-impregnated (pH 1) P-Ni-W/Al2O3 catalysts of low tungsten content (1.6 W atoms/nm(2)) and varying phosphorus concentration (0-5.9 wt.-% P2O5) have been studied in the reaction of hydrogenation of naphthalene in a batch Parr reactor at 573 K and P = 44 atm. In addition to a previous characterization by XRD, DRS and XPS, the catalysts have been characterized here by DRS, FTIR spectroscopy in their oxide state and by FTIR spectroscopy of NO chemisorbed on the sulphided catalysts. A promoting effect of phosphorus is observed in all phosphorus containing samples. The catalytic activity vs. phosphorus content curve has a complex behavior passing through a maximum and a minimum. Phase transitions are observed in the samples with the change of the phosphorus content. The form of the catalytic activity curve is discussed in the light of these phase transitions and the structure and properties of the different phases. The highest catalytic activity is observed for the sample with 0.6% P2O5. This sample is also characterized by the highest amount of chemisorbed NO and by a local maximum in the relative number of terminal W=O bonds. When strong overlapping of the bands is observed, the NO adsorption technique should be applied with care. Also, NO might adsorb on sites which are not active in the corresponding hydrotreatment reaction and the trend predicted by the total amount of chemisorbed NO should not necessarily coincide with the form of the catalytic activity curve. The catalytic activity curves indicate that during the reaction changes in the number of active sites or the formation of new active sites are possible.