Crystalline 1,3,6-trisubstituted uracils have been analysed by X-ray diffraction. 1,3,6-Trimethyluracil, C 7 H 10 N 2 O 2 (I), M r , = 154.17, triclinic, P1, a = 7.242 (2), b = 7.991 (3), c = 8.380 (3) A, α = 61.98 (3), β = 63.40 (3), γ = 82.09 (3)°, V = 381.2 (2) A 3 , Z = 2, D x = 1.343 Mg m −3 , λ(Cu Kα) = 1.54178 A, μ = 0.79 mm −1 , F(000) = 164, room temperature, final R = 0.060 for 1216 observed reflections. 1,3-Dimethyl-6-ethyluracil, C 8 H 12 N 2 O 2 (II), M r , = 168.20, monoclinic, P2 1 /n, a = 4.107(1), b = 9.806(1), c = 20.994 (2) A, β = 92.97 (1)°, V = 844.4 (4)A 3 , Z = 4, D x = 1.323 Mg m −3 , λ(Cu Kα) = 1.54178 A, μ = 0.76 mm −1 , F(000) = 360, room temperature, final R = 0.049 for 1284 observed reflections. 1,3-Dimethyl-6-propyluracil, C 9 H 14 N 2 O 2 (III), M r , = 182.22, monoclinic, P2 1 /c, a = 10.738 (1), b = 11.735 (1), c = 23.056 (2) A, β = 90.91 (2)°, V = 2904.9 (4) A 3 , Z = 12, D x = 1.250 Mg m -3 , λ(Cu Kα) = 1.54178 A, μ = 0.70 mm −1 , F(000) = 1176, room temperature, final R = 0.049 for 3048 observed reflections. 1,3-Dimethyl-6-butyluracil, C 10 H 16 N 2 O 2 (IV), M r = 196.25, monoclinic, P2 1 /c, a = 9.441 (6), b = 12.086(4), c = 18.751 (7) A, β = 100.53 (4)°, V = 2103 (2) A 3 , Z = 8, D x = 1.239 Mg m -3 , λ(Mo Kα) = 0.71073 A, μ = 0.08 mm -1 , F(000) = 848, room temperature, final R = 0.063 for 1812 observed reflections. In all structures, molecules are planar when only «heavy» atoms are considered. A layered-type structure was observed in all cases. In structures (III) and (IV), three and two independent molecules were found in the asymmetric unit, respectively. No conformational differences between the symmetrically non-equivalent molecules were detected. Correlations between thermochemical data and the properties of the compounds in the solid state are discussed