In situ measurements of air and water phases at a wastewater treatment plant (WWTP) were used to investigate the partitioning behavior of perfluorocarboxylic acids (PFCAs), perfluorosulfonic acids (PFSAs) and perfluorooctyl sulfonamide (HFOSA) and their conjugate bases (PFC ^-s, PFS ^-s, and FOSA ^-, respectively). Particle-dissolved (R _d) and air-water (Q _AW) concentration ratios were determined at different tanks of a WWTP. Sum of concentrations of C _4-12, ₁₄ PFC(A)s, C _4,6,8,10 PFS(A)s and (H)FOSA were as high as 50pgm ^-3 (atmospheric gas phase), 2300ngL ^-1 (aqueous dissolved phase) and 2500ngL ^-1 (aqueous particle phase). Particle-dissolved concentration ratios of total species, log R _d, ranged from -2.9 to 1.3 for PFS(A)s, from -1.9 to 1.1 for PFC(A)s and was 0.71 for (H)FOSA. These field-based values agree well with equilibrium partitioning data reported in the literature, suggesting that any in situ generation from precursors, if they are present in this system, occurs at a slower rate than the rate of approach to equilibrium. Acid Q _AW were also estimated. Good agreement between the Q _AW and the air-water equilibrium partition coefficient for C ₈PFCA suggests that the air above the WWTP tanks is at or near equilibrium with the water. Uncertainties in these Q _AW values are attributed mainly to variability in pK _a values reported in the literature. The WWTP provides a unique environment for investigating environmental fate processes of the PFCAs and PFSAs under 'real' conditions in order to better understand and predict their fate in the environment.