Quinazoline and phthalazine were hydrogenated at low pressure and temperature (60 psi, room temperature) and high pressure and temperature (2000 psi, 100°C, 150°C} in neutral and acidic solvents over 5% Pd/C, 5% Ru/C, 5% Rh/Al2O3 , 5% Rh/C, and 5% Pt/C. Chromatographic methods for determining the qualitative and quantitative composition of hydrogenation mixtures were developed. The compositions of quinazoline and phthalazine-hydrogenation mixtures were determined. Eight phthalazine-hydrogenation products were detected and isolated: (1) 1,2-dihydrophthalazine, (2) 1,2,3,4-tetrahydrophthalazine, (3) a,a'-diamino-o-xylene, (4) o-methylbenzylamine, (5) 1,3-dihydroisoindole, (6) o-xylene, (7 and 8) cis- and trans- 1,2-dimethylcyclohexane. These products were identified by isolating them from reaction mixtures and comparing their ir, nmr, and mass spectra with those of authentic samples. Pathways of hydrogenation were proposed for both quinazoline and phthalazine, and evidence for each pathway was presented.