Ion-selective electrodes (ISEs) with fluorous anion-exchanger membranes for the potentiometric detection of perfluorooctanoate (PFO-) and perfluorooctanesulfonate (PFOS-) were developed. Use of an anion-exchanger membrane doped with the tetraalkylphosphonium derivative (R f8(CH2)2)(Rf6(CH2) 2)3P+ and an optimized measurement protocol resulted in detection limits of 2.3 × 10-9 M (1.0 ppb) for PFO- and 8.6 × 10-10 M (0.43 ppb) for PFOS -. With their higher selectivity for PFO- over OH -, membranes containing the alternative anion exchanger (R f6(CH2)3)3PN+P((CH 2)3Rf6)3 with a bis(phosphoranylidene)ammonium group further improved the detection limit for PFO- to 1.7 × 10-10 M (0.070 ppb). These values are comparable with results obtained using well-established techniques such as gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and liquid chromatography-tandem mass spectrometry (LC-MS-MS), but the measurement with ISEs avoids lengthy sample preconcentration, can be performed in situ, and is less costly. Even when eventual spectrometric confirmation of analyte identity is required, prescreening of large numbers of samples or in situ monitoring with ISEs may be of substantial benefit. To demonstrate a real-life application of these electrodes, in situ measurements were performed of the adsorption of PFOS - onto Ottawa sand, which is a standard sample often used in environmental sciences. The results obtained are consistent with those from an earlier LC-MS study, validating the usefulness of these sensors for environmental studies. Moreover, PFOS- was successfully measured in a background of water from Carnegie Lake.