Dilute cyclohexane solutions of polybutadiene star polymers with 3, 4, 8, and 12 arms wherein one arm is perdeuterated have been investigated by small-angle neutron scattering. The scattering cross sections obtained showed a characteristic peak at finite values of the scattering vector, Q. This maximum is not a structure factor arising from intermolecular interactions but is intrinsic to the scattering law of star molecules with one arm of a different isotopic composition to the other arms. The theoretical scattering law developed for such isotopic copolymers was found to fit the observed scattering well and provided values of the radius of gyration of the labeled arm. No dependence of the radius of gyration on star polymer concentration was noted, but the radius of gyration of the labeled arm increased as the functionality of the star increased from 3 to 12, the increase in dimensions in the 12-arm star being ca. 25% greater than the radius of gyration obtained for the three-arm star. In these dilute solutions the radii of gyration were larger than obtained for the bulk melt state of the stars. The data have been compared with theoretical predictions, but generally a much stronger dependence of the radius of gyration on star functionality is observed than is predicted.