The equilibrium isotope effect (EIE) for the interconversion of the two chair isotopomers of 1-trideutero-1,3,3-trimethylcyclohexane was predicted using geometry and vibrational force constants derived from electronic structure theory at HF, B3LYP, and MP2 levels as input for the program THERMISTP. Agreement between theory and previously reported NMR results is very good (experimental K-eq = 1.042 +/- 0.001 vs K-eq = 1.0409 at MP2/6-311G* level, K-eq = 1.0503 at HF/6-311G*, and K-eq = 1.0417 at B3LYP/6-311G* level, all at 17 degrees C). In order to investigate the origin of this isotope effect, the calculated EIEs for the monodeuterated isotopomers were analyzed. It has been shown that the hydrogen atom on an axial methyl group which is unusually close to its counterpart on the other axial methyl is responsible for the large (steric) isotope effect in the compound studied.