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The molecular weight dependence of the radii of gyration Rg in a Θ solvent (trans-decalin) of one regular branched comb and three regular centipede polystyrenes was studied using a gel permeation chromatography system equipped with a two-angle light scattering detector and a refractive index detector. Rg in trans-decalin for each sample of particular molecular weight was about 25% smaller than that in a good solvent (tetrahydrofuran, THF). On the other hand, they are 20-40% larger than the theoretical values from the Gaussian chain model. This difference can be explained with the worm-like comb model developed by Nakamura et al. (Macromolecules 2000, 33, 8323-8328). Persistence lengths thus obtained for each sample were about half of that determined in THF solution. However, they are significantly larger than that for linear polystyrene. These results suggest that a main chain stiffening effect exists in comb polystyrenes even in a Θ solvent.