The anion complexation properties of a series of acyclic receptors consisting of diindolylurea groups appended with amide, amidoindole or amidocarbazole groups have been studied. The receptors selectively bind and encapsulate sulfate via either six or eight hydrogen bonds. Receptors containing eight hydrogen bond donors perturb the pK_a of bound dihydrogen phosphate and bicarbonate to the extent that they are deprotonated by free anion in solution.