The experimental results show that the decompositions of 2,2-dimethoxypropane (DMP) and 2,2-diethoxypropane (DEP) have nearly identical rate ratio of 2-methoxypropene (MPP) and 2-ethoxypropene (EPP) in ionic liquids. The yields ratio of MPP/methanol and EPP/ethanol were nearly 1.0. The four-center cyclic transition state was found in B3LYP density functional theory of level. Surprisingly, the experimental observations and theoretical rate constants ratio (k(DEP)/k(DMP)) appeared to differ about ten magnitudes at the experimental temperatures range. Further, dynamics study indicated that the significant tunneling effect played an important role in the identical rate constants ratio of the decomposition reactions at low temperatures range. (c) 2005 Elsevier B.V. All rights reserved.