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Mixed solvent systems for the critical liquid chromatography of polyisoprenes are investigated. Under critical solvent conditions, the energetic and entropic interactions between the polymer and the stationary phase cancel each other, the free energy of interaction approaches zero and the retention time is independent of the polymer chain length. Critical conditions for polyisoprene-silica gel have been established for a non-protic binary solvent mixture [i-octane-tetrahydrofuran (THF); critical composition at 0.88% (v/v) of THF] and a ternary system containing an acidic component [i-octane-[0.5% (v/v) acetic acid in methyl-tert.butylether (MTBE)]; critical composition at 0.65% (v/v) of the acetic acid-MTBE mixture].