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Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We study monolayers of noble gas atoms (Xe, Kr, Ar, and Ne) deposited on individual ultraclean suspended nanotubes. For this, we record the resonance frequency of the mechanical motion of the nanotube, since it provides a direct measure of the coverage. The latter is the number of adsorbed atoms divided by the number of the carbon atoms of the suspended nanotube. Monolayers form when the temperature is lowered in a constant pressure of noble gas atoms. The coverage of Xe monolayers remains constant at 1/6 over a large temperature range. This finding reveals that Xe monolayers are solid phases with a triangular atomic arrangement, and are commensurate with the underlying carbon nanotube. By comparing our measurements to theoretical calculations, we identify the phases of Ar and Ne monolayers as fluids, and we tentatively describe Kr monolayers as solid phases. These results underscore that mechanical resonators made from single nanotubes are excellent probes for surface science. © 2014 American Physical Society., We acknowledge support from the European Union through the Graphene Flagship (Grant No. 604391), the ERCcarbonNEMS project, and a Marie Curie Grant (No. 271938), the Spanish state (Grant No. MAT2012-31338), and the Catalan government (AGAUR, SGR). C.G. and J. B. acknowledge partial financial support from the Junta de de Andalucía Group PAI-205, Grant No. FQM-5987, MICINN (Spain) Grants No. FIS2010-18356 and No. FIS2011-25275, and Generalitat de Catalunya Grant No. 2009SGR-1003.