Light scattering measurements have been made on nine polyisobutylene (PIB) fractions in heptane, an intermediate solvent, at 25°C to determine the second virial coefficient A2 and the third virial coefficient A3 as functions of weight-average molecular weight Mw ranging from 2.7×104 to 7.8×106. For six of the fractions, z-average mean-square radii of gyration have also been determined. In the molecular weight range studied, A2 and A3 vary as Mw−0.21 and Mw0.58, respectively, and the factor g defined by A3/A22Mw is about 0.33 almost independent of molecular weight. The data of g and Ψ (the interpenetration function) as functions of the cube of the radius expansion factor αS, combined with previous data in cyclohexane, a good solvent, show that the two-parameter theory breaks down unless αS3 is larger than about 2 for g and about 5 for Ψ. It is concluded from the comparison of these g data with the recent theory of Norisuye et al. that the failure of the two-parameter theory for g at small αS3 is due primarily to the neglect of three-segment interactions and that the effect of chain stiffness on g in heptane is of minor importance in the Mw range studied. On the other hand, the stiffness effect on Ψ for PIB is found to be remarkable even at large αS3. In fact, the different dependences of Ψ on αS3 observed for heptane and cyclohexane solutions are explained semiquantitatively by the Yamakawa theory which takes account of the stiffness effect within the binary cluster approximation.