A comprehensive investigation of the reactions between diaminothiophosphinic chlorides, (R₂N)₂P(S)Cl (R = Me, Et, or Prⁱ), and aluminium trichloride confirms the potential for at least two modes of reactivity. Typical Lewis acid–base complexes have been observed at room temperature in solution by ³¹P and ²⁷Al n.m.r. spectroscopy. However, in the solid state, novel dimeric heterocyclic diphosphonium systems have been isolated for the Me₂N and Et₂N derivatives (crystal data for [{(Et₂N)₂PS}₂][AlCl₄]₂: monoclinic, space group P2₁/n, a= 10.598(2), b= 8.976(2), c= 19.370(4)Å, β= 98.65(2)°, Z= 2, R= 0.052). In contrast, the Prⁱ₂N derivative maintains the covalent Lewis acid–base adduct structure in the solid state [crystal data for (Prⁱ₂N)₂P(Cl)S·AlCl₃: monoclinic, space group P2₁/c, a= 12.705(5), b= 9.504(3), c= 18.016(6)Å, β= 92.85(3)°, Z= 4, R= 0.066]. The diaminoselenophosphinic chlorides show no evidence of adduct formation in solution; however, identical heterocyclic diphosphonium salts have been isolated in the solid state for the Me₂N and Et₂N derivatives (crystal data for [{(Et₂N)₂PSe}₂][AlCl₄]₂: triclinic, space group P, a= 10.635(7), b= 12.335(8), c= 15.159(9)Å, α= 95.94(8), β= 93.46(7), γ= 110.99(9)°, Z= 2, R= 0.066). The new heterocycles represent examples of heterocyclic thiophosphonium (and selenophosphonium) cations, and are structurally related to known neutral isovalent phosphetanes. In solution, the thiophosphonium salts dissociate and reform the Lewis acid–base adducts, while the selenium analogues adopt an equilibrium involving only ionic species. The delicate energetic balance between ionic and covalent structures is further demonstrated for the sulphur systems by the promotion of the ionic structures in solutions containing an excess of AlCl₃. However, the solution species react with CH₂Cl₂ by means of an electrophilic attack at the sulphur centre.