The reactions of the group 15 trihalides, MX₃ (M = As, Sb, Bi; X = Cl, Br), with the phosphine selenide SeP(p-FC₆H₄)₃ result in the formation of co-crystals of formula MX₃·SeP(p-FC₆H₄)₃. No reaction was observed with MI₃ (M = As, Sb, Bi). The structures of MX₃·SeP(p-FC₆H₄)₃ (M = As, X = Br 2; M = Sb, X = Cl 3; M = Bi, X = Cl 5; M = Bi, X = Br 6) have been established, and are isomorphous, crystallising in the cubic I23 space group. All the structures feature a primary MX₃ unit, which has three weak secondary MSe interactions to SeP(p-FC₆H₄)₃ molecules. However, each of these SeP(p-FC₆H₄)₃ molecules bridges three MX₃ molecules, resulting in the generation of an M₄Se₄ (M = As, Sb, Bi) distorted cuboid linked by the pnictogen–chalcogen interactions. Four opposing corners of the cuboid are occupied by the M atom (M = As, Sb, Bi) of an MX₃ pyramid, and the other four by the selenium atom of the phosphine selenide.