Diphenyl allenyl phosphonates are cyclized into 2,5-dihydro-1,2-oxaphosphol-2-ium ions in Brønsted acids (TfOH, H₂SO₄, FSO₃H). These species have been studied by means of ¹H, ¹³C, ³¹P NMR. The cations react with various nucleophiles (H₂O, MeOH, EtOH, Et₂NH, PhH) on the phosphorus atom. The obtained products are very labile and undergo decomposition. Contrary to Brønsted acids, under the action of Lewis acids AlX₃ (X=Cl, Br), these diphenyl allenyl phosphonates lead to another kind of heterocycles, 2,5-dihydrobenzo[f][1,2]oxaphosphepine-2-oxides.