AbstractIntrinsic viscosities have been obtained for polyisobutylene in the series of n‐alkane solvents at −12, 24.3, and 57.9°C. Values of the χ parameter have been calculated using [η]/[η]ϑ = α2.43 and the Flory closed expression relating α to χ (as modified by Stockmayer): α5 — α3 = 2 × 0.49 CMM1/2 [(1/2) — χ]. The values found by the first method are in excellent agreement with an approximate expression for χ derived using the Prigogine theory of polymer solutions, viz. Rχ = ArA/T + BT/rA where rA is the number of segments in the solvent molecule. Values of the constants A and B compare well with those found from heats of mixing of polyisobutylene with the same solvents. Values of χ are also obtained using the more recently Kurata‐Stockmayer‐Roig expression for the α — χ dependence although reasons are advanced for preferring the Flory‐Stockmayer equation. Qualitative, but less good quantitative agreement is now found between the χ values and the Prigogine theory.