3,5-Dinitrobenzoic acid, 1, forms a 2:1 molecular complex with 1,4-diiodobenzene, 2, through the formation of iodo-nitro interactions utilising one of its nitro groups, rather than forming a 1:1 molecular complex using both the nitro groups. The result suggests the importance of the hydrogen bond interactions present in the crystal of the parent molecule (1) in directing the interaction with Other donor molecules (2). (C) 1998 Elsevier Science Ltd. All rights reserved.