The electronic g-shift of FCl-(X(2)Sigma(+)) is calculated using second-order perturbation theory, a Breit-Pauli Hamiltonian and MRCI wavefunctions. Second-order contributions to Delta g (perpendicular to) are determined by the coupling with the two lowest Zn states. The Delta g (perpendicular to)-values generally increase linearly with R(F-Cl), but show a plateau near R-e. At equilibrium, the calculated values (in ppm) are Delta g (parallel to) = - 370, Delta g (perpendicular to) = 23 965, and (Delta g) = 15 855. Experimental values for FCl- trapped in crystals are -450 +/- 500, 27 400 +/- 2000, and 18 100 ppm, respectively. Allowing for crystal effects, the agreement is good. An optical absorption in the crystal at 4.13 eV is assigned to the transition 2(2)Sigma(+) <-- X(2)Sigma(+) (Delta E = 4.99 eV and f = 0.39).