A tube assembly (12) including an inner tube (13) having an outer thread (15), an outer tube (14) having an inner thread (16) engaging the outer thread (15), at least for securing an angular position between the inner tube (13) and the outer tube (14) is provided. Such a tube assembly should facilitate mounting and maintenance of a reverse osmosis system. To this end, the inner tube (13) includes at least an groove (17) on its outer surface and the outer tube (14) includes an recess (18) on its inner surface, wherein a blocking element is arranged in a space formed by the recess (18) and the groove (17) in overlapping relation with the groove (17) and the recess (18).