The synthesis of a double-cavity porphyrin with interesting allosteric binding properties toward viologens (N,N'-disubstituted 4,4'-bipyridines) is described. The porphyrin host forms very strong 1:2 complexes with viologens, displaying a negative allosteric behavior. The first viologen guest binds exceptionally tight (K > 107 M-1), and the second guest binds much more weakly (DeltaDeltaG = 9-15 kJ mol-1). The allosteric effect, one of the highest reported so far, originates in structural changes upon binding the first ligand, closely following the sequential (or induced-fit) theory of allosteric interactions by Koshland, Némethy, and Filmer (the KNF-model).