A cell has been built to measure the electrical resistivity of single crystals and thin films in a sulphur atmosphere. Resistivity variations during the sulphuration of pyrite samples have been investigated as a function of temperature ( approximately 450-723 K) and of time at constant temperature. The transformation of Fe thin films into pyrite films has been followed through their electrical resistivity changes up to approximately 623 K. Results are briefly discussed by considering the stoichiometric and crystallographic transformations undergone by the films. After these preliminary results some open questions are presented.