The reactions of the monomeric phosphanylboranes Ph₂P–BH₂·NMe₃ (1a) and tBuHP–BH₂·NMe₃ (1b) with the main group Lewis acids BH₃ and BBr₃ yield the adducts H₃B·Ph₂P–BH₂·NMe₃ (2a) and H₃B·tBuHP–BH₂·NMe₃ (2b), Br₃B·Ph₂P–BH₂·NMe₃ (3a), and Br₃B·tBuHP–BH₂·NMe₃ (3b). All these compounds were completely characterized by single-crystal X-ray structure analysis, NMR and IR spectroscopy as well as mass spectrometry and DFT computations. The secondary and tertiary phosphines 1a and 1b behave as classical Lewis bases to coordinate BX₃ (X = H, Br) as terminal groups.