The thermoelectric power of a series of isostructural organic charge-transfer salts has been measured. The compounds consist of a tetracyanoquinodimethane (TCNQ) anion with cations of tetrathiafulvalene, cis/trans-diselenadithiafulvalene, and tetraselenafulvalene. The salts are metallic at high temperatures with a sharp transition to a low-temperature insulating state clearly indicated by these measurements. In addition, the systematics of this study show the relative conductivities on the cation and anion chains and indicate similar scattering processes in all of these crystals. © 1976 The American Physical Society.