AbstractIn order to gain insight into the viscosity behavior of linear block copolymers, formulas have been developed for the calculation of radii of gyration of block copolymers whose blocks deviate from the theta condition by any given degree. In the course of this study the radii of gyration of sets of centrosymmetric arrays have been studied and the mean separation between the center of mass and the end of a random chain has been calculated. The results show that for AB block copolymers of fixed degree of polymerization (DP), the intrinsic viscosity varies in a monotonic, sigmoidal fashion when the composition is varied from homopolymer A to homopolymer B. The magnitude of the effect depends upon the relative stiffness of the A and B segments and upon the total DP. In ABA copolymers, on the other hand, at constant DP, varying the composition from all A to all B usually leads to a composition‐intrinsic viscosity relationship containing maxima and minima. As A becomes increasingly stiff compared to B, this relationship tends towards a monotonic one. This behavior is surprising and should be checked experimentall