The effect of carbon dioxide (CO2) sorption on the lower critical solution temperatures of deuterated polybutadiene/polyisoprene blends was determined with in situ small-angle neutron scattering. CO2 was a poor solvent for both polymers and exhibited very weak selectivity between the blend components. The sorption of modest concentrations of CO2, at pressures up to 160 bar, induced phase segregation at temperatures well below the binary-phase-separation temperature and caused an increased asymmetry in the lower critical solution temperature curve. The origin of solvent-induced phase segregation in this weakly interacting polymer blend system was attributed predominantly to an exacerbation of the existing disparity in the compressibility of the components upon CO2 sorption. (C) 2003 Wiley Periodicals, Inc.