Ruling party candidate Malam Bacai Sanha is seen as the front-runner
Guinea-Bissau has held a run-off vote to replace President Joao Bernardo Vieira, who was assassinated in March.
The poll pits two former heads of state against each other - Malam Bacai Sanha, seen as the favourite, and Kumba Yala.
Guinea-Bissau has a history of coups and its people say they are tired of broken promises and violence, a BBC correspondent reports.
President Vieira was killed in March in apparent revenge for the death of the head of the army in a bomb blast.
Mr Vieira led Guinea-Bissau for most of the period after independence from Portugal in 1974 - serving as president for a total of 23 years between 1980 and 2009.
There were no reported incidents of violence during Sunday's voting, and turnout among the 600,000 registered voters was estimated to be similar to the first round at around 60%.
'Time has come'
The first round of polling on 28 June saw Mr Sanha win nearly 40% of ballots, 10% more than Mr Yala, the AFP news agency reports.
When the two faced off in 2000, Mr Yala emerged as the winner.
In their final campaign rallies, both men repeated promises to bring peace and stability to the country.
Mr Sanha, who served as interim president from 1999-2000, is the candidate of the ruling PAIGC, the party of the 1970s struggle against Portuguese colonial rule.
This is the third time he has stood for president, having been defeated once by Koumba Yala and in 2005 by Mr Vieira.
His motto is "Hora Tchica" - meaning "the time has come".
Koumba Yala oversaw a period of economic crisis
Mr Yala, who was overthrown in a 2003 coup, is the leader of the opposition PRS.
Many Bissau-Guineans hold him responsible for changing the political and economic course of the country for the worse, the BBC's Luis Cardador says.
During Mr Yala's presidency, the IMF and the World Bank suspended aid to the country after accusations of mismanagement and a string of sackings in the government.
But he is believed to have wide support within the military.
Our correspondent says in past elections, voting has largely gone along ethnic or religious lines, but many people are now so fed up with the situation that this seems to be changing.
Guinea-Bissau is cash-starved and heavily dependent on just one product - the cashew nut.
In recent years it has become a major transit point in drug smuggling between South America and Europe.