Astrategy for the direct functionalization strategy of inertial dialkylp hosphonates with hydroxy compounds to affordd iverse mixed phosphonates with good yields and functional-group tolerance has been developed. Mechanistic investigations involving both NMR studies and DFT studies suggest that an unprecedented highly reactive P V species (phosphoryl pyridin-1-ium salt), ak ey intermediate for this new synthetic transformation, is generated in situ from dialkyl phosphonate in the presence of Tf 2 O/pyridine.