To determine the formation and reactivity of addition compounds produced during hydrodenitrogenation (HDN), we investigated the HDN of isoquinoline for a sulfided Ni-Mo/Al2O3 catalyst operated under a hydrogen pressure of 12 MPa (cold charge) in the temperature range 300-375 degrees C. The reaction products were classified into five groups of compounds: 1. hydrogenated derivatives of isoquinoline (tetrahydroisoquinolines, decahydroisoquinolines, and their isomers); 2. nitrogen-containing ring-opened products (1-amino-2-(2-methylphenyl)ethane and 1-amino-1-(2-ethylphenyl)methane); 3. denitrogenated products (1-ethyl-2-methylbenzene, 1-ethyl-2-methylcyclohexane, and their isomers); 4. addition products (hydrocarbons with molecular weights of 238, 244, and 250 and nitrogen-containing compounds with molecular weights of 249, 251, and 257); and 5. cracked products (toluene, ethylbenzene, dimethylbenzenes, and their hydrogenated derivatives). Most of the nitrogen-containing addition compounds appear to be substituted on the nitrogen atom. The HDN of isoquinoline was more than 10 times faster than the HDN of quinoline, whereas the hydrogenation of isoquinoline was difficult compared to the hydrogenation of quinoline. The reaction network for the HDN of isoquinoline is also presented.