Forms of surface species in molybdenum-nichel catalyst with high molybdenum loading have been studied by using the techniques of X-ray photoelectron spectroscopy, Laser Raman spectroscopy and temperature programmed reduction. The results indicate that the main species on surfaces of oxidic, reduced, sulfided catalysts are NiMo_xO_y whose structure is analo us to NiMoO_4, molybdates of different valence and metal nichel, molybdenum nickel oxysulfide and molybdenum sulfide respectively. The hydrodesulfurization activity sequence of the above three chemical states is: sulfided form, reduced form>>oxidic form and it is positively related to low temperature oxygen capacity. Metal nickel has been observed on the surface of reduced form. From this, it is proposed that spillover of hydrogen atoms created on metal nickel is one reason for the distinct increase of HDS activity in reduced catalyst. Surface species forms in sulfided, reduced states and HDS activities have been related. The results support the side-on catalytic mechanism proposed by H. Kwart.