The reaction of rare-earth nitrates with iminodiacetic acid (H(2)IDA) in the presence of cucurbit[6]uril (CB6) in water at room temperature yields a family of isomorphous complexes, [M(H2IDA)(NO3)(H2O)6](NO3)(2). CB6 center dot 14H(2)O with M = Y (1), Ce (2), Nd (3), Eu (4), Dy (5), Er (6), Tm (7), and Yb (8). In these compounds, the trivalent metal ion is bound to one nitrate ion, six water molecules, and one zwitlerionic H2IDA molecule. The latter is further partly included in the CB6 cavity, with the ammonium group forming two hydrogen bonds with oxygen atoms of one portal. The uncoordinated carboxylic group is thus encapsulated in CB6 and hydrogen-bonded to the other portal via a water molecule. CB6 is a second-sphere ligand in these complexes, but direct bonding of th 3 metal ion to CB6 can be enforced by using a ligand more deeply imbedded in the cavity, such as 2-pyridylacetate (PA) in [Eu(PA)(CB6)(NO3)(H2O)5liNO(3)) " 10H(2)O (11). When the reaction with H(2)IDA is performed with Lu(III) or Cu(II), no metal complex is isolated, but the inclusion compounds [(H(3)IDA)(2)(CB6)J(NO3)2. *120 with x = 6 (9) or 8 (10) are obtained instead, in which the two H3IDA+ cations are attached to the CB6 portals by ammonium carbonyl hydrogen bonds and are linked to one another inside the cavity by hydrogen bonding between the carboxylic groups. These complexes are compared to that comprising a dicarboxylic acid devoid of an ammonium functionality, [(H(2)AZ)(CB6)].6H(2)O (12), where H(2)AZ is azelaic acid. The metal ion complexes 1-8 and the organic complexes 9 and 10 display the unprecedented feature of inclusion of carboxylic groups inside the CB6 cavity, with the CB6/ acid stoichiometry and the finer details of the host guest interactions being dependent on the presence of the metal ion.