Sao Tome votes for new president
The July 17 poll will pit former president Manuel Pinto da Costa against current parliament speaker Evaristo de Carvalho, opposition LSTP-PSD leader Aurelio Martinez and six others.
Early results could emerge late on Sunday and a poll run-off will be held if no candidate wins an outright majority of ballots from the 92,000 registered voters. Observers said polling was taking place peacefully early on Sunday.
Sao Tome and Principe is so far failing to convince oil majors to invest in its energy sector after initial excitement faded because of disappointing results from wells.
The government is also keen to develop tourism on the island characterised by largely unspoilt, palm fringed beaches. It also has a small cocoa sector with a reputation for high-quality beans.
Opinion polls published in local media put da Costa and de Carvalho ahead. Da Costa, 74, an economist, was the former Portuguese colony's president from independence in 1975 until 1991, running a socialist one party state in the country.
Current President Fradique de Menezes is nearing the end of his second five-year term so he cannot stand.