In this paper, we present the use of metathesis catalysts in solutions of nitrile butadiene rubbers (NBR). In general, metathetic degradation induces chain scission along a polymer backbone having unsaturated bonds. More specifically, the reaction is used to obtain tailored Mooney viscosities of nitrile rubbers with optimal molecular weight and molecular weight distribution for a given application. When metathesis is applied to the rubber which is then subjected to hydrogenation, hydrogenated nitrile rubbers (HNBR) with Mooney viscosities of 60 MU and even below can be obtained. We optimized the reaction conditions of various commercially available catalysts with different chemical structures to achieve high efficiency and found out that the optimum condition varied surprisingly between the types of catalyst. The influence of the nature of NBR feedstock and reaction temperature on the progression of the reaction are also described. The most noteworthy effect occurs when adding additives, such as salts and Lewis acids, because they can be both beneficial and detrimental to the efficiency of the catalyst.