The chemical state of the active phase (Ni-Mo-S and Ni-W-S) of sulfided NiMo and NiW catalysts during the interaction with NO is investigated by extended X-ray absorption fine structure (EXAFS). The results show that approximately 30 % of the Ni atoms in the active phase are oxidized already at room temperature (RT) upon NO exposure. On the average, the chemical state of the W atoms in sulfided NiW/Al2O3 is hardly changed. Essentially all the Ni atoms in the active Ni-Mo-S phase are oxidized above 150 degreesC. Although the present results indicate that significant structural and chemical changes occur when NO is used as a probe molecule in activated hydrotreating catalysts, NO temperature-programmed desorption (TPD) and NO chemisorption can, nevertheless, provide important insight into the nature of the active phase.