The separate contributions of the TTF and TCNQ chains to the total magnetic susceptibility of TTF-TCNQ are evaluated from measurements of the gyromagnetic ratio and EPR linewidth. From the temperature dependence of these susceptibilities, we show that the metal-insulator transition at 54°K affects primarily the TCNQ chain. The TTF chain is primarily affected by the transition at 38°K and dominates the conductivity in the semiconducting regime.The electronic properties of TTF-TCNQ have been described 1 in terms of two partially filled bands corresponding to the donor and the acceptor chains. Each of these bands makes a contribution to the measured physical properties such as conductivity 2 * 3 and susceptibility. 4 " 6 In general, the role of each chain in determining the magnitude of a given property is difficult to delineate because the respective contribution of the donor or acceptor stack is modified by the presence of the other. The only known exceptions are the gyromagnetic ratios of the TTF and TCNQ chains which were shown 7 to maintain their intrinsic g values in TTF-TCNQ. Moreover theseg values are very different, 8 ' 9 since the TTF molecule has a much bigger 10 spin-orbit coupling than TCNQ.In the present paper we shall show how the magnetic susceptibility can be separated into TTF and TCNQ contributions using EPR^ values 11 and linewidth measurements. The temperature dependence of the individual susceptibilities in the temperature range which includes both the 54 and 38°K phase transitions is used to evaluate the respective effects of these transitions on the donor and acceptor chains. The 54°K transition is shown to affect primarily the TCNQ chain, while the TTF chain is primarily affected by the 38°K transition. The separation of the susceptibility for 20°K8 and the g value 8 of the (TTF) + cation in solution. This fact, combined with the activated nature of the susceptibility, 4 " 6 clearly indicates that the measured susceptibility in this temperature re-gion is all on the TTF stack, presumably because the magnetic gap on the TCNQ stack is greater than the magnetic gap on TTF. Similar behavior 13 was observed in TMTTF-TCNQ (TMTTF is tetramethyl-TTF) and also 14 for some compositions of the isostructural series (TSeF) x (TTF) 1 . 3e -(TCNQ) (TSeF is tetraselenafulvalene), 15 where 0^ x^ 1. However the magnetic gap on the acceptor chain depends on the nature of those chains. For example, the reported 16 g-values for TTF-TNAP (TNAP is 11,11,12, indicate that the magnetic gap in the semiconducting regime is smaller on the acceptor chain than on the donor chain. The separate donor and acceptor contributions to the total susceptibilit...

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