Ethyl orthoformate reacted with pinacol to form 2-ethoxy-4,4,5,5-tetramethyl-1,3-dioxolan (I) which decomposed on heating to yield ethanol, carbon dioxide, and 2,3-dimethylbut-2-ene. When subjected to the same reaction sequence diethyl- D-tartrate, diethyl-meso-tartrate, meso-1,2-diphenylethane-1,2-diol, and cis-cyclo-hexane-1,2-dial underwent specific cis-elimination to yield diethyl fumarate, diethyl maleate, cis-stilbene, and cyclohexane respectively. Glycerol, butane-1,2,4-trial, and pentane-1,2,5-triol formed polymers which decomposed to yield allyl alcohol, but-1-en-4-ol, and pent-1-en-5-ol respectively. Pyrolysis of the monoformyl esters of cis-cyclohexane-1,2-diol, glycerol, butane-1,2,4-triol, and pentane-l,2,5-trio1 gave the respective products enumerated above whereas the monoformyl ester of pinacol yielded pinacolone. When compound (I) was heated in the presence of p-toluenesulphonic acid it gave ethyl formate, pinacolone, 2,3-dimethylbutadiene, and water.