The investigation was formally opened on Wednesday. The allegations, that the two bids agreed to back each other in the separate contests for 2018 and 2022 respectively, are being looked at by ethics committee chairman Claudio Sulser.
FIFA general secretary Jérôme Valcke said the collusion allegations were first brought to his attention several weeks ago. The committee will also consider what role may have been played by executive committee members allied to Qatar and Spain.
Vote trading is in breach of FIFA rules and could result in both or either bid being thrown out of the contest.
On Tuesday the ethics committee voted unanimously to suspend Nigeria's Amos Adamu and Oceania football chief Reynald Temarii, the two FIFA executive members at the centre of cash-for-votes allegations.
