The intramolecular nucleophilic addition of N-methyl-N-(2,4,6-trinitrophenyl)glycine anion in methanol-dimethyl sulfoxide mixtures produces spiro[(3-methyl-5-oxazolidinone)-2,1'-(2',4',6'-trinitrobenzenide)]. The spiro adduct has been identified by means of 1H and 13C NMR spectroscopy. This is the first case when the formation of a Meisenheimer adduct with carboxylate ion is observed. Logarithm of the equilibrium constant of adduct formation increases linearly with the mole fraction of dimethyl sulfoxide in its mixture with methanol.