The effect of using citric acid in the synthesis of unsupported tungsten phosphide (WP) hydrotreating catalyst was investigated by synthesizing WP using varying amounts of citric acid. Catalyst characterization was done using nitrogen physisorption, CO chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The activities of the catalysts were tested on a high pressure kinetics rig using the hydrodenitrogenation of pyridine as a test reaction. The use of citric acid in the preparation resulted in smaller WP particles-1-10 mu m versus >100 mu m when no citric acid was used. The characterization results suggest that the citric acid formed complexes with the W ions and limited the agglomeration of these ions during calcination. The smaller WP particles were more active catalysts for pyridine conversion on both per volume and per site basis. (C) 2009 Elsevier B.V. All rights reserved.