Effects of the temperature and heating rate in the calcination step over some properties of Ni/HZSM-5 catalysts have been studied and discussed. The catalysts have been characterized by temperature-programmed re ducti on, ammonia temperature-programmed de sorption, X-ray diffraction line broadening, and X-ray photoelectron spectroscopy (XPS) techniques, and n-decane hydroconversion has been employed as a test reaction. Changes in the strength acidity distribution of the catalysts with the calcination temperature were observed. XPS measurements revealed that the nickel precursor compound Ni(NO3)(2) decomposes totally in the catalysts calcined above 573 K, whereas a small proportion of the precursor was detected in the catalysts calcined at 573 K. This fact as well as the reduction of the different existing metallic species determines the minimum particle size for the catalysts calcined at 723 K. The reaction parameters in n-decane hydroconversion also showed the effect of the calcination temperature.