The thermolysis of 1-[4-(phenylethyl)benzyl]naphthalene, NMBB, was studied under different reaction conditions. The conversion of NMBB is mainly a result of -CH2-CH2- bond cleavage in the presence of high pressure D-2. However, in the presence of 9,10-dihydrophenanthrene the fraction of N-p-CH2 bond (N-p = naphthyl) cleavage increased significantly. The sulfided Co-Mo-alumina catalyst, which gives more than 30% conversion of NMBB even at temperatures as low as 350 degrees C, increased the fraction of NMBB that is converted through N-p-CH2- bond cleavage to more than 0.9. Possible roles for solvent and catalyst are proposed and alternative mechanisms for the thermolysis of NMBB are discussed.