Phosphines are often less basic than corresponding amines, for instance the phosphonium ion itself has a pKa of −14 compared to 9.21 for the ammonium ion; trimethylphosphonium has a pKa of 8.65 compared to 9.76 for trimethylammonium.
As fosfinas son menos básicas que as aminas correspondentes, por exemplo, o ión fosfonio ten un pKa de -14 en comparación con 9,21 para o ión amonio; o trimetilfosfonio ten un pKa de 8,65 fronte a 9,76 para a trimetilamina.