The performance of commercial hydrotreating units that use 15% LCO/85% SRGO as feed stock and a MoNi/Al2O3-type catalyst to produce 30-ppm sulfur diesel was followed along the cycle length. The products generated at the 1st, 8th and 14th months on stream were deeply characterized to enable understanding of the changes in quality along the cycle length. Three catalyst samples at the end of run were obtained from the reactor and characterized. Furthermore, a WNiPd/TiO2Al2O3 catalyst, deactivated in the pilot plant during 3 months in operation with 100% LCO as feedstock, was then used to process the same feedstock as in the commercial unit. A diesel engine test was done with the three products obtained along the cycle length as well as with others obtained in the pilot plant to check the effect of deactivation on emissions. The results confirmed metal and acid site deactivation, decrease of cetane number and increase in emissions along the cycle length. The test with a synthetic feed demonstrates that the metal sites on the catalysts deactivated more slowly than the isomerization and cracking ones. (C) 2007 Elsevier B.V. All rights reserved.