According to music critic Robert Christgau, "Crystalised" uses a musical structure similar to "VCR", "hooky ostinato riff embellished with a few subtle effects, leading to a soft vocal. The exaggerated drawl of bassist Oliver Sim precedes the more human-scale croon of guitarist Romy Madley Croft." Croft and Sim shared lead vocals, which Rachel Kowal from NPR said are "delivered in a sexy call and response fashion that is reminiscent of the duets" by Canadian indie pop band Stars on their 2004 album Set Yourself on Fire, "minus all the antagonism associated with ex-lovers."
Like most of the other songs on xx, "Crystalised" was based on an imagined romantic scenario, developed from a verse Sim had written thinking about what a fling would be like: "I hadn't really had any relationships to be working off. But I had a huge interest in life, and looking at other people’s relationships around me." He wrote the first lines of the song, including "you applied the pressure / to keep me crystalised", and shared it through iChat with Croft, who responded with her verse, beginning with "I'll forgive and forget / Before I’m paralyzed." John Colapinto of The New Yorker critiqued that "the finished lyrics, some of their best, suggest a twisted love affair, but also hint at drugs and ecological disaster."
"Crystalised" was first released in the UK on 27 April 2009 by Vinyl and on 25 November 2009 by Digital download and then again on 11 October 2010. It has been used in numerous advertisements and TV shows and has over 26 million hits on YouTube as of June 2011. It was sampled in the song "Less Than Zero" by K.Flay and covered by Gorillaz whose version was included in the advertisement for Grey's Anatomy in late 2010. In 2013, it was covered and released as a single by Martina Topley-Bird, Mark Lanegan, and Warpaint.