[2] According to Robert Christgau, "Crystalised" uses a musical structure similar to "VCR", "hooky ostinato riff embellished with a few subtle effects, leading to a soft vocal.

"[3] 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.

"[4] 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.

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.