Anions rather than cations: While resorcinarenes bind cations through cation–π interactions, methylene-bridged cavitands surprisingly bind anions through C[BOND]H⋅⋅⋅anion interactions with the acetal protons at the wider rim (see model of the complex in the scheme). Mass spectrometry, in agreement with theory, provides evidence for anion binding on the concave side of the cavitand bowl.