The efficient, simple and environmentally friendly synthesis of pentaerythritol diacetals and diketals using SO3H-functionalized ionic liquids (ILs) as catalysts was reported. The ILs showed high catalytic activities and reusabilities with good to excellent yields of the desired products. H-0 (Hammett function) values and the minimum-energy geometries of SO3H-functionalized ILs were determined and the results revealed that the acidities and catalytic activities of ILs in acetalization were related to their structures. The IL [PSPy][OTf] with the shortest H-O bond distance had the strongest acidity and the highest catalytic activity in the synthesis of pentaerythritol diacetals and diketals. (C) 2010 Elsevier BM. All rights reserved.