Tensimetric studies on the reactivity of cationic oxo- (I[BArF₂₄]) and thiophosphonium (II[BArF₂₄]) Lewis acids towards the reference Lewis bases acetonitrile and pyridine are reported ([BArF₂₄]⁻ = tetrakis[2,6-bis(trifluoromethyl)phenyl]borate). I[BArF₂₄] forms a thermally stable 1:1 complex with pyridine and a solvate with acetonitrile, while II[BArF₂₄] forms a labile complex with pyridine and a solvate with acetonitrile. The crystal structure of the acetonitrile solvate of I[BArF₂₄] was determined by X-ray structural analysis. At elevated temperatures, reactions of perfluorinated anions with the glass of the apparatus were observed. Quantum chemical computations indicate that the Lewis acids undergo significant reorganization upon complex formation with pyridine. The stability of the cationic complexes with respect to dissociation decreases in the order I⁺·Py > II⁺·Py > I⁺·AN > II⁺·AN. Solvation in benzene and acetonitrile decreases dissociation energies of the complexes by 5 – 20 kJ mol⁻¹. Acetonitrile appears to be a suitable solvent for the reactivity studies of cationic oxo- and thiophosphonium Lewis acids.