Three-dimensional X-ray crystal structure analyses have been carried out on the compounds MIH[C5H4BrN2O3]2. For MI= K, Z= 4 in a monoclinic unit cell having a= 14·884, b 14·779, c= 7·135 Å, β= 97·61°, and space group l2/c. The cation lies on a two-fold axis which relates two anionic moieties with a short [2·42(2)Å] hydrogen bond between oxygen atoms. The potassium is ten-co-ordinate and K–O distances range from 2·739–3·105(9)Å, mean 2·944 Å; each uracil chelates two potassium ions and makes one contact with a third. These K–O contacts and the ‘symmetrical’ hydrogen bond hold the structure in columns along the needle axis of the crystal, c. Hydrogen bonds N–H O, 2·73(1)Å, hold the columns together in the other two directions. For M1= Rb, Z= 4 in an orthorhombic unit cell having a= 5·995, b= 21·825, c= 11·932 Å and space group lba2. The rubidium ion is on a two-fold axis and is six-co-ordinated by a trigonal prism of oxygen atoms at distances from 2·93–3·00(1)Å, mean 2·977 Å. The three oxygen atoms of one uracil moiety make contact with different rubidium ions and each oxygen with only one rubidium. Metal–oxygen contacts and hydrogen bonds [N–H O 2·88(3) and O O 2·42(5)Å] hold the structure in layers; shortest contacts between the layers are Br Me 3·59 and Br Br 3·68 Å. The shorter hydrogen bond joins N(3) oxygen atoms across a two-fold axis. The differences in structure are correlated with differences in the i.r. spectra.