The first X-ray crystallographic studies are reported for water-soluble inclusion complexes of the macrocyclic p-sulfonatocalix[5]arene(1) (1b). The complexes Na-7[p-sulfonatocalix[5]arene]. 18H(2)O (2) and Na-5[p-sulfonatocalix[5]arene]. py-N-O . 8.5H(2)O (3) containing water and pyridine N-oxide (py-N-O) guest species have been isolated. In 2 the large bowl-shaped cavity of the host molecule is occupied by a total of three water molecules, while for 3 a single molecule of pyridine N-oxide is included and interacts with the host via a strong hydrogen bond to a protonated sulfonate substituent. The behavior of 1b as a ligand for a number of lanthanide elements is also reported. Low temperature X-ray structural studies reveal that coordination of the Ln(3+) metal ions to the calixarene ligand occurs exclusively via the sulfonato functionalities and not the phenolic oxygen atoms as suggested by Shinkai et al. for the complexation of UO22+ by 1b. In complex 2 the calixarene exists as the heptaanion. In contrast, crystallization from acidic solution results in complexes 3-8 retaining all their phenolic protons. The protonation constants of 1b have been measured in aqueous solution : pK(a) = 10.96(8), 7.63(5), and 4.31(1) at 25 degrees C and 0.1 M KNO3.