The self-assembly into nanotubes in solution of an amphiphilic copolymer is characterized by small-angle neutron scattering (SANS). This study confirmed the shape and the size of the tubular association and the 3-D association of the tubes predicted by molecular orbital theory. Moreover, the characterization of the stability of the association has revealed that the addition of a very small amount of salt to the solution increases the size of the association. When more salt is added, the size of the association decreases, and the structure is altered. The association was found to be independent of temperature and therefore is very stable.