Five different signals have been analyzed in ESR spectra obtained from sulfided molybdenum- or tungsten-containing catalyst samples. Signal I (oxo-Mo5+, g = 1.933 for Mo/¿-Al2O3; and oxo-W5+, g = 1.78 for W/¿-Al2O3), and possibly signal III arise as a result of interactions with the support. Signal II (g = 1.985 for Mo/SiO2, and g = 1.91 for W/¿-Al2O3) and signal IV (g = 1.995 for W/¿-Al2O3, and g = 2.01 for WS2 bulk) have been detected both on supported and on unsupported sulfided samples. These two signals show a complementary behavior upon evacuation and H2S adsorption and are therefore ascribed to paramagnetic surface species in the MoS2 and WS2 phases. Some surface configurations are proposed to describe the origin of these paramagnetic surface species. The origin of signal V which has been detected in supported and unsupported samples is still unknown.