The crystal structures of 1: 1 adducts of cuprous iodide with collidine ICu(col) and 2,6-lutidine ICu(lut), and of cuprous chloride with pyridine ClCu(py), have been determined at 295 K and refined by full-matrix least squares to residuals of 0.052 [ICu(col)], 0.079 [ICu(lut)], 0.057 [ClCu(py)] for 465, 1761 and 720 ‘observed’ reflections respectively. Crystal data.-ICu(col), orthorhombic, P21mn, a 11.500(3), b 10.620(3), c 4.159(1) Å, Z 2. ICu(lut), monoclinic, P21/c, a 16.377(9), b 7.833(3), c 15.401(8) Å, β 110.53(4)°, Z 8. ClCu(py), monoclinic, P21, a 9.324(4), b 3.8484(7), c 8.663(4) Å, β 107.08(3)°, Z 2. The 1: 1 cuprous bromide-pyridine adduct BrCu(py) is isomorphous with ClCu(py): a 9.423(2), b 3.9585(6), c 8.907(2) Å, β 108.13(2)°. In none of the three structures is the tetrameric (CuXL)4 species found. The structure of ICu(col) is an infinite zig-zag I-Cu-I-Cu-polymer running parallel to c; the copper is also coordinated by the collidine, Cu, I and N lying in the mirror y = 0.5 which also bisects the collidine ring. The structure of ICu(lut) comprises dimeric species, a pair of three-coordinated copper atoms being bridged by the two associated iodines. ClCu(py) has the ‘ribbon’ structure typified by CuCl, MeCN and CuBr, MeCN, an infinite polymer parallel to b. The inability of ICu(col) and ICu(lut) to form tetramers is attributed to steric hindrance of the methyl substituents of the base. The Cu-Cu distance in ICu(lut) is remarkably short (2.586 Å).