In this work, the effect of substituents on the valence states of biferrocenium salts was investigated in order to explore the valence control of ionic molecular materials. Reactions of 1′,1‴-disubstituted biferrocene derivatives (R2-bifc) and fluoro tetracyanoquinodimethanes (Fn-TCNQ; n = 1, 2, 4) produced single crystals of six salts (1–6), most of which were either monovalent ([D]+[Am]−; m = 1–3) or divalent ([D]+[Am]−) salts. (Et2-bifc)(F1-TCNQ)2 (1) was a mixed-stack monovalent salt, whereas the corresponding F2-TCNQ salt was a divalent salt. (MeO2-bifc)(F1-TCNQ)2 (2) was a monovalent salt because of the small Madelung energy of its segregated-stack structure, despite the low redox potential of the donor. (MeS2-bifc)(2,6-F2-TCNQ)2 (3) was a mixed-stack salt with an apparent intermediate valence state ([D]1.5+[A2]1.5–), containing [D]2+ and [D]+ in a 1:1 ratio. Its valence state, which is intermediate between those of corresponding salts with F1-TCNQ and 2,5-F2-TCNQ, is probably related to the unsymme...