The translational diffusion coefficient D was determined from dynamic light scattering measurements for oligo- and polyisobutylenes in isoamyl isovalerate (IAIV) at 25.0 degrees C (Theta) and in n-heptane at 25.0 degrees C in the range of weight-average molecular weight M(w) from 1.01 x 10(3) to 1.76 x 10(6). The values of the unperturbed and perturbed hydrodynamic radii RH(H,Theta) and R(H) defined from D in IAIV and in n-heptane, respectively, were found to agree with each other in the oligomer region, indicating that the values of R(H,Theta) may be adopted as those of the unperturbed hydrodynamic radius R(H,0) in n-heptane at 25.0 degrees C. The values of the hydrodynamic-radius expansion factor alpha(H) in n-heptane at 25.0 degrees C are then obtained as the ratio R(H)/R(H,Theta) from those values thus determined. The data for R(H,Theta) are analyzed as usual by the use of the corresponding (unperturbed) helical wormlike (HW) chain theory. The results for alpha(H) as a function of the scaled excluded-volume parameter (z) over tilde defined in the Yamakawa-Stockmayer-Shimada theory for the HW chain with excluded volume are consistent with the previous results for atactic polystyrene and poly(dimethylsiloxane). The implication is that the quasi-two-parameter scheme may be valid for alpha(H) as well as for the gyration-radius and viscosity-radius expansion factors alpha(s) and alpha(eta) irrespective of the differences in chain stiffness, local conformation, and solvent condition. It is again found that the Barrett equation overestimates alpha(H). This disagreement between theory and experiment may be qualitatively explained by the Yamakawa-Yoshizaki theory, which takes account of the possible effect of fluctuating hydrodynamic interaction on alpha(H). It is also again found that alpha(H) coincides with alpha(eta) within experimental error over the whole range of M(w) studied.