The final vote was down to Rio and Madrid, Spain. In earlier voting, Chicago, Illinois, and then Tokyo, Japan, were eliminated from contention.
After the cities made their presentations to the IOC on Friday, the IOC members sat down to cast their votes in a secret ballot.
Ninety-seven of the IOC's 106 members are eligible to vote in the first round; seven must sit out that round because they represent one of the countries bidding for the Games.
A city must receive a majority of votes in order to win. If no city receives a majority, voting moves on to a second round, with the city receiving the lowest number of votes being eliminated, an IOC spokeswoman said. source