Hexamethoxy-calixarene has been fully functionalized with p-phosphonic acid groups on the upper rim in57% yield over three steps and has been authenticated in the solid state by X-ray diffraction as either a nitrate salt or one of two calcium complexes. The latter differ by the ratio of calcium ions per calixarene, either 3:1 or 4:1. In both structures, the coordination sphere of the calcium ions is made up of oxygen atoms from the phosphonic acid groups and from water of crystallization, as part of extended polymeric layers in the extended three-dimensional packing. Hirshfeld surface analysis shows extensive O⋯H and O⋯Ca interactions for the phosphonic acid moieties in both calcium structures. Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) of the hexaphosphonic acid shows nanoarrays consisting of up to a maximum of 28 calixarene units.