Novel spirocyclic compounds of formula I: and pharmaceutically acceptable salts thereof are disclosed as inhibitors of the ROMK (Kir1.1) channel. The compounds may be used as diuretic and/or natriuretic agents and for the therapy and prophylaxis of medical conditions including cardiovascular diseases such as hypertension, heart failure and chronic kidney disease and conditions associated with excessive salt and water retention. Pharmaceutical compositions and methods of treatment are also included.