A series of highly dispersed single phase pentlandites (Co9S8, NiCo8S8, FeCo8S8, Ni3Co6S8, Ni4.5Fe4.5S8) was prepared by coprecipitation followed by sulfidation and reducing treatment at 300 degreesC. These mixed sulfides consist of crystalline particles with homogeneous distribution of the elements as shown by high-resolution microscopy and EDX analysis. The distribution of the cations in tetrahedral and octahedral sites of the pentlandite structure was characterized by EXAFS and Mossbauer spectroscopy. The catalytic activity of the sulfides in the model thiophene hydrodesulfurization reaction increases in the following order: Ni4.5Fe4.5S8 much less thanFeCo(8)S(8) < Co9S8 < NiCo8S8 < Ni3Co6S8. Activity -properties correlations are discussed, on the basis of extended Huckel electronic structure calculations and temperature programmed reduction measurements.