TMET-TTP derivatives in which one or both of the outer 1,3-dithiole rings are replaced by 1,3-diselenole rings (1a-1d) have been prepared (TMET-TTP: 2-[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-5-(4,5-ethylenedithio-1, 3-dithio1-2-ylidene)-1,3,4,6-tetrathiapentalene). The neutral crystal of 1c has β-type uniform stacks, whereas la shows a Θ-type molecular arrangement in the neutral crystal. The AsF6 salts of 1a and 1d have a Θ-type structure similarly to other TMET-TTP salts. In spite of the relatively small dihedral angle (117°), the conductivity is weakly semiconductive from room temperature. Selenium substitution causes unbalance of the diagonal intermolecular interactions, which may be the origin of a deviation from the universal phase diagram of the Θ-phase.