The isostructural series of alloys (TSeF)x(TTF)1-xTCNQ is investigated with the intention of better understanding tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and TSeF-TCNQ and their several surprising differences. For several compositions, the crystal-lattice parameters are measured by indexing the x-ray powder patterns and the transitions are characterized by means of the dc conductivity versus temperature. The dominance of the TCNQ stacks and the unimportance of fulvalence disorder in producing the metal-insulator transition are confirmed for 0≤x0.9. The lowering of the transition temperature for this range is attributed to the change in lattice parameter along the stacks and the associated change in TCNQ transfer integral and charge transfer. The importance of the fulvalene stacks for 0.9x≤1 is shown by an anomalously high sensitivity of the transition temperature to fulvalene doping. Possible explanations for this anomalous behavior are considered. © 1978 The American Physical Society.