The reduction of allenyl(diphenyl)phosphine oxides with HSiCl₃ or LiAlH₄ selectively afforded the corresponding allyl(diphenyl)phosphine oxides. 3-Methylbut-2-en-1-yl(diphenyl)phosphine oxide reacted with AlCl₃ to give a mixture of 4,4-dimethyl-1-phenyl-1,2,3,4-tetrahydro-λ⁵-phosphinoline 1-oxide and 4,4-dimethyl-1-phenyl-1,4-dihydro-λ⁵-phosphinoline 1-oxide.