Results or the lirsr quanIiurive experimen~zd rests of [he theorericnl predictions of differeminl line broadening (DLB) in coupled spin-l/t systems are reporcsd Tar the PFOS-anion. These prediclions. based on the Redfield dewily matrix theory_ agree well with the enperimenml observations. The absolute sign of the spin coupling constnnr is shown 10 be nrgztive I,., = -87O.Ort:OS Hr The experimenml conditions needed for chc monireslntion or DLB in coupled spin sgslems nrr given. 1. ~troduction Differential nuclear magnetic resonance (NMR) line broadening in coupled spin-l/Z, spin-l/l, AX systems was discussed by Shin&u some years ago [I] *_ This work, which is based on the density matrix formalism of Redfield [3], also pointed out the possibility of obtaining from straightforward experiments the absolute sign of the AX indirect spin-spin coupling constant. The first experimental test of this theory was done by Mackor and MacLean [4] _ They carried out a proton and fluorine relaxation study of CHFCI,, and showed that the absolute sign of the HF coupling in this molecule was positive_ A complete quantitative test of the theory was not possibIe because of the presence of the chlorine nuclei and the lack of symmetry of the molecule. These and other aspects of NMR relaxation in coupled spin systems have been recently reviewed by Void and Vold [S] _ We report here the results of phosphorus-3 1 and fluorine-19 transverse relaxation studies of fluorophosphate anion, PFO$-. To the best of our knowledge these results represent the first clear, careful, quantitative test of differential NMR line broadening for mobile species. Such a test is possible only if the ' Conicil Fellow -Venezueln. * Some of the results of ref. [ 11 have been refo~ulated by Goldman [ 21.