A LaNiO3 perovskite-type oxide catalyst precursor was prepared by the autocombustion method. The catalysts were submitted to a different process of H2S poisoning, before and after reduction, as well as during reforming reactions, using different H2S concentrations (0-250 ppm). Deactivation becomes faster by increasing the H2S concentration. The materials before and after the ex situ and in situ poisoning processes were characterized by X-ray diffraction. In all cases, the metal and support were severely affected. Nickel sulfides with different stoichiometry (NixSy), lanthanum oxysulfide (La2O2S), and lanthanum sulfide (LaSy) were observed.