The reaction of tBu(C₆H₄O₂)P, with the borane B(C₆F₅)₃ gives rise to NMR data consistent with the formation of the classical Lewis acid–base adduct tBu(C₆H₄O₂)P(B(C₆F₅)₃) (1). In contrast, the NMR data for the corresponding reactions of tBu(C₂₀H₁₂O₂)P and Cl(C₂₀H₁₂O₂)P with B(C₆F₅)₃ were consistent with the presence of equilibria between free phosphine and borane and the corresponding adducts. Nonetheless, in each case, the adducts tBu(C₂₀H₁₂O₂)P(B(C₆F₅)₃) (2) and Cl(C₂₀H₁₂O₂)P(B(C₆F₅)₃) (3) were isolable. The species 1 reacts with PhCCH to give the new species tBu(C₆H₄O₂)P(Ph)CCHB(C₆F₅)₃ (4) in near quantitative yield. In an analogous fashion, the addition of PhCCH to solutions of the phosphines tBu(C₂₀H₁₂O₂)P, tBuPCl₂ and (C₆H₃(2,4-tBu₂)O)₃P each with an equivalent of B(C₆F₅)₃ gave rise to L(Ph)CCHB(C₆F₅)₃ (L = tBu(C₂₀H₁₂O₂)P 5, tBuPCl₂6 and (C₆H₃(2,4-tBu₂)O)₃P 7). X-Ray data for 1, 2, 6 and 7 are presented. The implications of these findings are considered.