The interaction in air of benzyltrimethylammonium chloride with uranyl sulphate in methanol solution, containing sulphuric acid, has provided a variety of products one of which we have now characterized as benzyltrimethyl-ammonium µ₂-peroxo-bis[trichlorodioxouranate(VI)], containing one of the first examples of a complex containing a dioxygen molecule, bonding as a µ₂-peroxo-linkage. The crystals are monoclinic, P2₁/c, with a= 8.869(5), b= 11.013(5), c= 25.60(1)Å and β= 103.7(1)°. The asymmetric unit contains [N(PhCH₂)Me₃]₂[UO₂Cl₃(O₂)_½], Z= 4. An automatic Weissenberg diffractometer was used to measure 2 442 independent reflections using MO-K_α, radiation at room temperature. The structure was refined by full-matrix least squares to R= 0.114 and R′= 0.072. The anion consists of two distorted coplanar Cl₃O₂ pentagons. They surround the uranyl groups which lie perpendicular to the plane of the pentagons and which are linked by a bridging oxygen molecule in which both oxygen atoms are equidistant from both uranium atoms.