A WNiPd/TiO2Al2O3 mild hydrocracking catalyst deactivation has been studied in a small pilot plant unit. OTMHCK technology can produce low sulfur, high cetane number diesel at constant conversion with acceptable cycle length by increasing initial temperature as a function of time on stream. Three sets of tests were performed for 3 months at constant operating temperature using the same catalyst. At the start and at the end of the run the catalyst activity and selectivity were tested at different temperatures. Spent catalyst was characterized using chemical analysis, C-13 NMR, XPS, and ammonia adsorption techniques. The hydrocracking reactions were modeled using a lump model including 63-equation reactions, an empirical reactivity function and three distribution functions that included hydrogenation, cracking, and ring opening in liquid phase. Deactivation reaction was simulated using a Levenspiel's two sites type of deactivation mechanism. A Genetic Algorithm tool was used to obtain the deactivation constant using a set of previously determined kinetic constants. This set takes into account the aromatics adsorption on metal and acid sites. The results confirm that deactivation changes the relative rate of hydrogenation/hydrogenolysis, cracking and dealkylation reactions. (c) 2005 Elsevier Ltd. All rights reserved.