5- and 6-membered heterocyclic compounds which are inhibitors of phosphodiesterase 10 are described as are processes, pharmaceutical compositions, pharmaceutical preparations and pharmaceutical use of the compounds in the treatment of mammals, including human(s) for central nervous system (CNS) disorders and other disorders which may affect CNS function. Also described is the treatment of neurological, neurodegenerative and psychiatric disorders including but not limited to those comprising cognitive deficits or schizophrenic symptoms.