The fragmentation behaviour of the ion MeP(O)OMe⁺ has been investigated using quantum mechanical calculations at the B3LYP and MP2 levels to support experiments made with an Ion Trap Mass Spectrometer. Two mechanisms for the loss of CH₂O are found, one involving a 1,3-H migration to phosphorus and the other a 1,2-methyl migration to give P(OMe)₂⁺ followed by a 1,3-H migration. In each case an ion-dipole complex is formed that rapidly dissociates to yield CH₂O. The relative importance of each route has been previously determined experimentally via isotopic labelling experiments, and the theoretical results are found to be consistent with these experimental results. The mechanisms suggested in the earlier work involving a 1,4 H migration to O are shown to be energetically unfavourable.