Calculations of isotope effects for the gas-phase reaction of silylene with acetaldehyde have been carried out using a combination of ab initio, transition state, and RRKM theories. Ab initio calculations were used to provide optimized structures at the HF/6-31G(d) level along the reaction coordinate at fixed (SiO)-O-... distances. These structures were matched to previously measured reaction A factors (via entropies of activation), to find the transition state structures (geometries and vibrational wavenumbers) appropriate to each temperature at which previous rate constant measurements were made. Transition state calculations were then made to obtain the isotope effect k(1,H)k(1,D) for the bimolecular addition process. RRKM calculations were then undertaken which gave the isotope effect pressure dependence, at the pressures of the experimental study. The calculated overall isotope effects, k(H)/k(D), which were only slightly temperature and pressure dependent, lay in the range 1.005-1.252. These are in good agreement with experimental values. Uncertainties arising from reactant orientation are discussed. The reason for the small isotope effects arises from a compensation of contributing factors due to the looseness of the transition state.