A new thallium(I) uranate(VI) of composition Tl4UO5 was isolated and characterised by X‐ray, i. r. and chemical analyses. The compound dissociated into thallium(I) oxide and Tl2UO4 on heating to 230°C and its subsequent thermal behaviour depended on the volatilisation and oxidation characteristics of the thallium(I) oxide released. Copyright © 1974 Verlag GmbH & Co. KGaA, Weinheim