Reaction of phosphine oxides R3P=O [R = Me (1a), Et (1c), Pr-1 (1d) and Ph (1e)], with the bromophosphoranimines BrPR'R '' P=NSiMe3 [R' = R '' = Me (2a); R' = Me, R '' = Ph (2b); R' = R '' = OCH2CF3 (2c)] in the presence or absence of AgOTf (OTf = CF3SO3) resulted in a rearrangement reaction to give the salts [R3P=N=PR'R '' O-SiMe3]X (X = Br or OTf) ([4]X). Reaction of phosphine oxide 1a with the phosphoranimine BrPMe2=NSiPh3 (5) with a sterically encumbered silyl group also resulted in the analogous rearranged product [Me3P=N=PMe2O-SiPh3]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl substituted phosphine oxide, (Bu3P)-Bu-t=O (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized phosphoranimine cations [(Bu3P)-Bu-t=O center dot PR'(2)=NSiMe3](+) ([3](+), R' = Me (d), OCH2CF3 (e)). A mechanism is proposed for the unexpected formation of [4](+) in which the formation of the donor-stabilized adduct [3](+) occurs as the first step.