The structure of 4,4'-bipyridinium diiodide has been determined. The diprotonated cation possesses crystallographic inversion symmetry with the pyridyl rings coplanar. The crystal packing is dominated by weak but directional N-H···I (N···I distance of 3.47(2) Å) and C-H···I (C···I distances of 3.74(3)3.77(3) Å) interactions to give layers of ions. The layers stack parallel to the (101) direction and are linked together by an additional C-H···I interaction (C···I distance of 3.816(5) Å). Crystal data: triclinic, P-1 (No. 2), a = 8.2940(13), b = 8.3842(11), c = 5.0978(8) Å, a = 93.761(11)(degrees), (beta)= 112.281(12)(degrees), ? = 106.887(11)(degrees), V = 307.63(8) Å3, Z = 1, Scalc = 2.22 g/cm3. Final residual values based on 997 observed (I > 2s(I)) reflections: R = 0.026, Rw = 0.034.
