The synthesis is described of a series of monofunctionalized tetrathiafulvalene (TTF) derivatives, by reactions of tetrathiafulvalenyllithium with a range of electrophiles: substituents are thereby attached to the TTF frame by carbonyl, ester, thioester, amide, and thioamide groups. The X-ray crystal structures of four TTF derivatives are described, three of them for the first time. These are 4-[O-(4-chlorobutyryl)thiocarboxyl]-TTF(9), 4-[N-methylthioamido]-TTF(10),4-[N-phenylamido]-TTF (11), and 4-[N-phenylthioamido]-TTF(12). The electron-withdrawing ability of these substituents influences the geometry of the TTF ring. Within all four structures the shortest intermolecular S --- S and S --- C contacts are coplanar with, or slightly inclined to, the TTF planes. The structure of compound 10 provides the first example of kappa-phase packing in a neutral TTF donor.