An unexpected [1,4]-phospho-Fries rearrangement that gives rise to the formation of a O,O,O,O-tetraethyl methylenebis(thiophosphonate) derivative is reported. The regioselectivity of the metallation with n-BuLi or t-BuLi is the key factor that explains either the [1,4] or [1,3] rearrangement observed.