While processing lower sulfur levels in diesel fuel force increasing demands for gas oil deep HDS in the refineries, problems of refractory sulfur compounds and inhibiting/competitive retarding effects in hydrodesulfurization reactions gain growing importance. High performance sulfur and carbon-specific capillary gas chromatographic analysis combined with computer-aided chromatogram evaluation has been applied as a powerful tool for determination of the feedstock processability and the investigation of gas oil composition and the course of reactions. Gas oil matrices have been found as of high influence on the reaction rates of sulfur compounds. Reaction rates have been measured and correlated predominantly with the total initial sulfur content of the gas Oil. The inhibiting species an visualized as strongly adsorbed remarkably stable intermediates, which an formed through partial hydrogenation of multi-ring sulfur compounds.