"I.O.I.O." is a song by the Bee Gees, released on the album Cucumber Castle. It was written by Barry and Maurice Gibb. The song was released as a single in March 1970, and was also one of the highlights of the album. The single was a relative success mainly on European charts. Its music video is taken from the film Cucumber Castle.

It was a cult favourite of fans, however, and made it onto the compilation Best of Bee Gees, Volume 2.

The title "I.O.I.O." is derived from the calls on the chorus sung by Maurice accompanied by Colin's drumming, and Barry sings the verses and plays guitar. Former Bee Gees guitarist Vince Melouney was featured, having played the guitar on the song's first session on 12 June 1968 during sessions for, Idea, but was not present when the song was finished, as he left the group following the album Odessa. The song's second session was October 8, 1969 after they had recorded "Twinky" (unreleased) and "The Chance of Love". (Pentangle drummer Terry Cox added drums on the October 8 sessions) Maurice claims that the song was not even quite finished, but it still had Barry's guide vocal on it. The song also is notable for being the only Bee Gees A-side single to feature any vocal solos from Maurice Gibb, these being on the "I.O." chant.

It marked the group's first conscious delving into what is now called 'world music'. According to Robin Gibb, it grew out of Barry's visit to Africa.Maurice Gibb described this as "Barry's African jaunt". This is evident from the percussion break at the song's beginning.

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