Functional chemical modifications on poly(ethylene terephthalate) (PET) fibers via radical graft polymerization could be controlled by managing mutual interactions and affinities between different components in the grafting reaction system. Hansen solubility parameters was used as a tool to quantify affinities of related agents and the polymer, and provided reliable results. The latest results proved the practicality of using Hansen solubility parameters in controlling radical graft polymerizations on surface modifications of PET fibers. Four different monomers with different hydrophilic properties in different solvent and initiator systems were examined, and results confirmed that interactions of initiator‐PET, initiator‐solvent, monomer‐PET, monomer‐solvent, and monomer‐initiator play important roles in determining the grafting reaction efficiency. Results revealed that for the selected grafting systems studied, hydrophilic monomers presented overall favoring affinities toward PET leading to higher grafting yields compared to hydrophobic monomers. The results have instructive impact to commercial applications. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45990.