Dimethyl hemiorthoformate, the putative intermediate in the methanolysis of methyl formate, has been detected by 1H n.m.r. spectroscopy as an intermediate in the hydrolysis of dimethoxymethyl acetate in solution in mixtures of [2H6]acetone (90–86 volumes) and deuterium oxide (10–14 volumes) at –35 °C. Similar experiments with 2-acetoxy-1,3-dioxolan lead to the detection of 2-hydroxy-1,3-dioxolan and with 2-acetoxy- and 2-chloroacetoxy-4,4,5,5-tetramethyl-1,3-dioxolan to 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan. The last compound was also detected by 13C n.m.r. spectroscopy. The relative rates of breakdown of dimethyl hemiorthoformate, 2-hydroxy-1,3-dioxolan, and 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan were approximately 1, 0.3, and 0.03. The breakdown of 2-hydroxy-4,4,5,5-tetramethyl-1,3-dioxolan was shown to be acid catalysed with a second-order rate constant 29.4 l mol–1 s–1 in [2H6]acetone–deuterium oxide (9:1 v/v) at –40 °C.