Polymer-polymer interaction energy densities have been measured by two different techniques: inverse gas chromatography (IGC) and gas sorption experiments in a Cahn electrobalance. Both use a third component as a molecular probe to investigate the interactions between the two polymers. The poly(epichlorohydrin)/poly(methyl acrylate) (PECH/PMA) blend has been selected as a system that ensures the liquid state of the polymeric phase in all the experiments. It also stresses the convenience of a previously proposed procedure that selects the most adequate solvents to obtain a reliable polymer-polymer interaction energy density by IGC. A permanent gas, CO2, has been used as molecular probe in sorption measurements, after verifying the inconvenience of using organic solvents as probes, due to the overshoot phenomena observed in their sorption kinetics. Both techniques give similar polymer-polymer interaction values.