An experimental study of the non‐Newtonian viscosity μ and the principal normal stress differenceN1of solutions of polybutadiene, polystyrene, and their copolymers of varying microstructures has been carried out in decalin, decane, and their mixtures. Decane is a nonsolvent for polystyrene and a poorer solvent for polybutadiene than decalin. For the homopolymers and random copolymers, the viscosity and Weissenberg number(N1/σ12)at a constant shear rate decrease as the nonsolvent is added. For block copolymers, the opposite is observed. This is due to the polystyrene segments precipitating out in poor solvents and forming intermolecular aggregates. The SBS‐block copolymers tend to form viscoelastic gels because a three‐dimensional network with polystyrene crosslinks is formed. The simple SB‐block copolymers show thixotropic behavior with yield stress. This may be explained by the two‐phase structure being in the form of micelles in which the polystyrene aggregate are solubilized by the polybutadiene segments.