The empirical polarity parameter E-T(30) was determined for polyethylene terephthalate (PET) fiber and five phosphorus flame retardants by UV-Vis absorption spectrum of solvatochromic probe. To investigate the interaction between flame retardant and PET fiber during the finishing process, the partition coefficients K of these flame retardants between water and PET fiber were determined and an interesting linear relationship between lnK and E-T(30) of the flame retardants was obtained, suggesting E-T(30) is definitely an excellent estimation of the interaction of flame retardants with PET. Additionally, the thermodynamic criterion of this finishing process, standard Gibbs free energy of transfer of the flame retardants from water to PET fiber, Delta G(t)(0) (FR,H2O -> PET), was found to linearly correlate with E-T(30) of the flame retardants as well, Delta G(t)(0) (FR,H2O -> PET) = 0.3664*E-T(30)(FR) - 18.60 (R-2 = 0.9168), demonstrating polarity of flame retardant is an important and determining factor for its fixation on PET fiber. This work may provide theoretical guidance for choosing flame retardant finishing agent more compatible with PET fabric. (C) 2017 Elsevier Inc. All rights reserved.