Second-order rate constants for the demethylation of methyl di(3-nitrophenyl)phosphinate by iodide in acetone-d6at 25 °C have been measured. For a series of iodide salts the rate increased in the order: NBu4I < KI ≤ NaI < LiI. The addition of crown ethers or D2O has a retarding effect on the reaction rate. The results are discussed in terms of the increase of the nucleofugality of the phosphinate group via the complexation of metal ions with the phosphoryl function.