The Allen–Millar–Trippett rearrangement is a ring expansion reaction in which a cyclic phosphine is transformed into a cyclic phosphine oxide.^[1] This name reaction, first reported in the 1960s by David W. Allen, Ian T. Millar,^[2] and Stuart Trippett,^[3] occurs by alkylation or acylation of the phosphorus, followed by reaction with hydroxide to give a rearranged product. The hydroxide first attacks the phosphonium atom, followed by collapse to the phosphine oxide with one of the groups migrating off of the phosphorus.