Fluorine kinetic isotope effects (F KIEs) have been determined using the accelerator-produced short-lived radionuclide F-18 in combination with natural F-19. The solvent dependence of the leaving group F KIE was investigated for the nucleophilic aromatic substitution reaction (SNAr) of 2,3-dinitrofluorobenzene with piperidine at 22 degrees C. The F KIE was determined to be 1.0262 +/- 0.0007 in tetrahydrofuran (THF) and 0.9982 +/- 0.0004 in acetonitrile. The significant F KIE in THF suggests that, in this solvent, departure of the leaving group is the rate-limiting step. On the other hand, the almost vanishing KIE in acetonitrile is consistent with addition of the nucleophile to the aromatic substrate being the rate-limiting step. A full account of the previously communicated (Matsson et nl. J. Am. Chem. Sec. 1993, 115, 5288-5289) experimental method for determination of fluorine KIEs is also given.