The luminescent behaviour of uranyl (UO₂²⁺) in aqueous colloidal silica solution is illustrated and its biexponential decay with time investigated. The two components have lifetimes of 240 and 55 µs and after about 1.5 h the decay becomes monoexponential with a lifetime of 240 µs and luminescence quantum yield approaching 25%. The absorption and emission spectra are red shifted and greatly enhanced compared with those in dilute aqueous perchloric acid solution. The suitability of aqueous colloidal silica as a dispersed medium for enhancing luminescence to detectable levels for concentration measurements is also shown, with uranyl concentrations down to 10 nmol dm^–3(0.2 ppb) being routinely measurable. The luminescence of uranyl on dried silica gel is found to be almost identical with that of uranyl in aqueous colloidal silica, illustrating that the uranyl is extremely closely associated with the silica surface and thereby sheltered from quenching by water molecules. This colloidal enhancing medium has advantages over traditional methods used in that a longer lifetime is achieved, diffusional quenching by anions is considerably reduced and the uranyl can be easily extracted from bulk solution.