The experimentally observed stereomutation of spiro-1,2-oxaphosphetanes is shown by DFT calculations to proceed through successive M-B2 or M-B4 and M-B3 mechanisms involving two, four, and three Berry pseudorotations at phosphorus, respectively. Oxaphosphetane decomposition takes place in a single step via a polar transition state. The calculated activation parameters for this reaction are in good agreement with those determined experimentally.