This was recorded in a one-off session with the producer Jacknife Lee just after the 2007 Carling Weekend: Reading and Leeds festivals.
Kele Okereke told the New Musical Express October 20, 2007: "I don't think it's a radically different direction. There's a quite noticeable guitar sound and it's still got the real anthemia thing we do. I don't see it being any different from a track such as 'The Pioneers.' It just felt like a Bloc Party song with keyboards. We wanted to write a real high-energy, euphoric dance tune, something you would hear at Trash or Trade. So instead of writing a huge guitar track, we thought we'd go the whole way. I mean, we didn't want to write 'Banquet' again.
It's about trying to recognize that a moment has passed between people. It can apply to personal and professional relationships. It's the hardest thing recognizing that something that was there isn't any more."
Okereke revealed in a 2007 interview with BBC 6 Music that Flux's edgier dance feel was not an indication that Bloc Party were looking to go in a new direction. He said: "'Flux' isn't an indication at all about the next record. The important thing about this record for me was that it showed we could do anything we wanted, we weren't going to settle for being any other guitar band of 2007. Luckily it was successful." Drummer Matt Tong added: "I think one crucial thing about that song was it was radically different from anything we attempted before and I think it was important in terms of opening up... broadening our horizons."
According to online reports, "Flux" was originally offered to Kylie Minogue for her X album but she turned it down.
The song's music video was directed by Ace Norton (Norah Jones and Death Cab for Cutie). The clip was filmed over two days in Boston with American performance troupe Kaiju Big Battel and pays homage to Japanese kaiju movies. "There's something really moving about the robots falling in love," said Okereke to NME in 2013. "Even now, when I see it, it makes me feel sentimental."