The dimensions and configurations of polyisobutylene (PIB) chains in the bulk and in a θ-solvent have been investigated by elastic neutron scattering at small and at intermediate angles. The z-average radii of gyration, 〈s2〉 1 2z, of p rotonated PIB were calculated from Zimm plots in the Guinier region. Values of 〈s2〉 1 2z measured in a deuterated PIB matrix (75±5 Å) and in the θ-solvent (77±5Å) are in good agreement. Results of scattering measurements at intermediate angles were compared using Kratky plots on an absolute scale. The scattering curves for protonated PIB in the deuterated PIB matrix and in the θ-solvent were in excellent accord in the range of 0<μ<0.6Å-1 (μ = 4πsinθ/λ), demonstrating that the configuration in the bulk is virtually the same as in a θ-solvent. The experimental data were compared also with theoretical scattering functions. Close agreement throughout the range of μ covered by the experiments confirms the underlying conformational analysis and demonstrates randomness of configurations in the bulk down to distances of ca. 10 Å. © 1983.