Hydrogen isotope effects on geometries, total energies, nuclear and electronic wave functions of the [HO3SO–H–OSO3H]− and [KO3SO–H–OSO3K]− complexes are investigated with the NEO/HF method. This method determines both electronic and nuclear wave function simultaneously. A discussion of the isotope effects is provided and used to explain the hydrogen isotope effects on the phase transition temperatures in hydrogen bonded ferroelectric materials, K3H(SO4)2 and K3D(SO4)2.