A theoretical study based on bidimensional model surfaces that retains the main features of the potential hypersurfaces has been performed in order to analyze the influence of primary and secondary effects on intramolecular proton transfer tunneling frequencies of malonaldehyde in the gas phase. To this aim, several isotopically substituted species have been considered. The tunneling frequency is calculated and compared for the different species. It is found that substitution of the transferring proton by deuterium or tritium greatly diminishes the tunneling whereas isotopic substitution of hydrogens not directly involved in the transfer has no appreciable effect when the symmetry of the double well is preserved. However, these secondary effects dramatically increase only by the introduction of a slight asymmetry in the double well potential by isotopic substitution. All these results agree with previous experimental data on isotopically substituted malonaldehyde species. © 1992, American Chemical Society. All rights reserved.