Infinite-dilution activity coefficients of 26 solvents in three polybutadienes were determined by inverse gas chromatography in the temperature range 40-100°C. The polybutadienes have well-characterized structure and different molecular weights. The data are useful for the development of thermodynamic models for polymer-solvent interaction and for characterization. The results were fitted to the PHCT equation of state. The data indicate that activity coefficients depend on polymer molecular weight and structure.