In 1856, Burton was asked to make a voyage to East Africa, to find the sources of the Nile. He again chose Speke as his companion. The two travelled inland from Zanzibar and discovered Lake Tanganyika. They heard of a second lake in the area, but Burton was too sick to make the voyage. Speke thus went alone, and found the lake, which he christened Lake Victoria.
Speke returned to England before Burton, and made their voyage famous. Burton was embittered, because Speke declared Lake Victoria to be the Nile's source, whereas Burton believed Lake Tanganyika to be so, and because Speke had by then already been chosen to lead an expedition to further clarify the issue.
Together with James Augustus Grant, Speke left from Zanzibar in October 1860. They travelled on the west side around Lake Victoria without actually seeing much of it, but on the north side of the lake, Speke found the Nile flowing out of it and discovered the Rippon Falls. Next he travelled to Gondokoro in southern Sudan, where he met Samuel Baker, then back to England.
Speke's voyage did not resolve the issue, Burton claimed that because Speke had not followed the Nile from the place it flowed out of Lake Victoria to Gondokoro, he could not be sure they were the same river. A debate was planned between the two on September 16, 1864, but Speke died just one day before, of a hunting accident - although Burton and some others believed it might actually have been suicide.