Bis(1,3-diphenyl-l,3-propanedionato)oxo-(pyridine)uranium(VI) (1), [UO2(CasHllO2)2(CsHsN)], Mr= 795.6, monoclinic, P2Jn, a= 10.158 (2), b= 21.937 (8), c= 13.888 (4)A, t= 103.49 (1) ° , U = 3009 (2)/k 3, Z = 4, D x = 1.76 g cm -3, 2(Mo Ka) = 0.71069 A, /t = 51.48 cm -1, F(000) = 1536, T= 289 K, R = 0.036 for 3269 unique observed reflections. Bis[ 1-(tert-butoxy)-1,3-butanedionato]dioxo-(pyridine)uranium(IV) (2), [UO2(CaHI303)2(CsHsN)], M r= 663.5, orthorhombic, Cmc2~, a = 15.939 (9), b = 19.532 (8), c= 9.777 (6) A, U= 3044 (1) A 3, Z = 4, D x = 1.45 gcm -a, 2(Mo Kct) = 0.71069/~, /t = 50.81 cm -1, F(000) = 1280, T= 289 K, R = 0.048 for 1178 unique observed reflections. Dioxobis(2,4-pentanedionato)(pyridine)neptunium(VI) (3), [NpO2(CsH 7-O2)2(CsHsN)], Mr=546.4, orthorhombic, Fdd2, a = 29.617 (5), b= 11.406 (2), c= 10.595 (4)A, U= 3579 (2) A 3, Z=8, D x=2.03gcm -3, 2(MoKa)= Acta Cryst. Abstract.