The mean-square radius of gyration [S-2] was determined from small-angle X-ray scattering and light scattering (LS) measurements for oliga- and polyisobutylenes (PIE) in n-heptane at 25.0 degrees C in the range of the weight-average degree of polymerization x(w) from 1.14 x 10 to 2.48 x 10(3). The model parameters of the helical wormlike (HW) chain for PIE are then determined by analyzing simultaneously the present data for [S-2]/x(w) and the previous data obtained from LS measurements for samples with large x(w) in the same solvent condition and also in isoamyl isovalerate at 25.0 degrees C (Theta), by the use of the HW theories with and without the excluded-volume effect. With the parameter values so determined for PIE, the plots of the viscosity- and hydrodynamic-radius expansion factors alpha(eta) and alpha(H) against the scaled excluded-volume parameter (z) over tilde for PIE along with those for atactic polystyrene and atactic and isotactic poly(methyl methacrylate)s are shown to form a single-composite curve better than before in each case, confirming the validity of the quasi-two-parameter scheme. The improvement of the consistency of the results for PIB with those for the other polymers arises from the re-evaluation of (z) over tilde with the present values of the model parameters directly determined from [S-2].