In concentrated aqueous hydrobromic medium, Na[AuCl4] reacts with 1,6,7-trimethyllumazine (1,6,7-trimethyl-pteridine-2,4(1H,3H)-dione, MLMD) to give an adduct with simplified formula (H7O3)[AuBr4](DLMD)3 in which the negative charge of the tetrabromoaurate(III) anion is neutralized by an oxonium dihydrate cation (H7O3 +). The compound has been characterized by means of analytical techniques and IR and NMR spectroscopies. The crystals belong to the triclinic P —1 space group, with a = 11.321(1), b = 11.908(2), c = 16.460(1) Å, α = 71.54(2), β = 71.675(7), γ = 78.68(1)°, Z = 2, R = 0.089. The structure is formed by uncoordinated pteridine derivatives hydrogen-bonded, in a ladder-like polymeric fashion, to trinuclear oxonium-dihydrate cations; free square-planar tetrabromoaurate(III) anions are allocated in the holes of the H-bond network.