A non-parametric analysis using the Fredholm integral equation of the first kind exploits the distribution of the catalytic active sites during the hydrodenitrogenation (HDN) of carbazole on nitrided and reduced 12.5% Mo/Al2O3 catalysts. The approach was combined with the reactivity data and the surface property measurements. The equation describes two kinds of active sites (Site 1 with a small rate coefficient and Site 2 with a high rate coefficient) on the catalysts. The 1173 K-nitrided catalyst had a high activity and large Site 1 population along with a very low population and the highest value of Site 2, while the treatment at 773 K with NH3 or H-2 produced less Site 1 than Site 2. The 973 K-nitrided catalyst had the highest k(2).x(2) value of the four catalysts, showing that it was superior to the other catalysts for Site 2. Based on the reaction product selectivity, Site 1 of the nitrided catalyst was responsible for the hydrogenation of carbazole, while Site 2 was responsible for the C-N hydrogenolysis of perhydrocarbazole. The turnover frequency (carbazole disappearance rate divided by the CO adsorption) was correlated to the k(1)x(1) value, but not to the k(2)x(2) value, showing that Site 1 facilitated the HDN reaction controlled by the carbazole hydrogenation. The two sites and mechanism for the carbazole HDN are discussed. (c) 2009 Elsevier B.V. All rights reserved.