We report the synthesis and encapsulation properties of long (up to 5 nm) molecular nanotubes 1-4, which are based on calix[4]arenes and can be filled with multiple nitrosonium (NO+) ions upon reaction with NO 2/N2O4 gases. These are among the largest nanoscale molecular containers prepared to date and can entrap up to five guests. The structure and properties of tubular complexes 1·(NO +)2-4·(NO+)5 were studied by UV/Vis, FTIR, and 1H NMR spectroscopy in solution, and also by molecular modeling. Entrapment of NO+ in 1·(NO +)2-4·(NO+)5 is reversible, and addition of [18]crown-6 quickly recovers starting tubes 1-4. The FTIR and titration data revealed enhanced binding of NO+ in longer tubes, which may be due to cooperativity. The described nanotubes may serve as materials for storing and converting NOx and also offer a promise to further develop supramolecular chemistry of molecular containers. These findings also open wider perspectives towards applications of synthetic nanotubes as alternatives to carbon nanotubes. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA.