During flash cooling of protein crystals in liquid cryogens, cooling rates are determined by sample size, choice of cooling liquid, and by the thickness of the cold gas layer that forms above the liquid. We describe an experimental protocol for ultra-rapid cooling of protein crystals. This protocol requires no complex apparatus, and yields ice-ring-free diffraction without the use of penetrating cryoprotectants. © 2007 Springer Science+Business Media B.V.