The invention pertains to tricyclic heteraaryi compounds that serve as effective phosphodiesterase (PDE) inhibitors. The invention also relates to compounds which are selective inhibitors of PDE 10. The invention further relates to pharmaceutical compositions comprising such compounds; and the use of such compounds in methods for treating certain central nervous system (CNS) or other disorders. The invention relates also to methods for treating neurodegenerative and psychiatric disorders, for example psychosis and disorders comprising deficient cognition as a symptom.