The dithallous salt of 2,4-dihydroxy-2,4-dimethylpentanoic acid (1), formed by slow diffusion of acetone into an aqueous thallous cyanide solution, is transformed into 2,4-dihydroxy-2,4-dimethylpentanoic acid (2) by reaction with ammonium iodide. 2-Hydroxy-2,4,4-trimethylbutyrolactone (3) is obtained by treatment of 2 with catalytic amounts of benzenesulfonic acid. The sequence of the reactions and the crystal structure of 1 are discussed.