Abstract The reaction of monocyclic oxyphosphoranecompounds 1a, 1b, 1c, with ethylene glycol in pyridine was studied by 31P NMR. The results showed that compound 1a, with an unsaturated five-membered ring reacts slightly faster than compound lb with a saturated ring attached by two trans p-nitro phenyl groups, which reacts 100 times faster than the cis compound lc. To interpret the mechanism, hexacoordinated