We present a comparative study of the magnetic susceptibilities of TSeF-TCNQ (tetraselenafulvalene-tetracynaoquinodimethane) and its isostructural sulphur analogue TTF-TCNQ (tetrathiafulvalene-TCNQ). The overall temperature dependences of the two susceptibilities are similar, but notable differences in detail are seen. In the selenium compound the donor (TSeF) contribution to the susceptibility is much less than that of the acceptor, indicating a larger bandwidth and/or smaller enhancement. In contrast to the case of TTF-TCNQ, in TSeF-TCNQ we find the same activation energy for magnetic excitations as for single-particle excitations in the insulating state. The alloy series TTF1-xTSeFxTCNQ, is found to interpolate smoothly between the two extremes. © 1978 The American Physical Society.