Multiconfiguration self-consistent-field calculation has yielded the dipole moment function for the X1Σ+ state of HI, which qualitatively confirms the experimental finding that the dipole derivative at Re is negative. The calculated dipole moment for the v = 0 vibrational level is 0.665 D for both HI and DI as compared with the experimental values of 0.38 and 0.445 ± 0.02 D, respectively. The calculated potential curve yields values of Re, D0, and ΔGv+ 1 2, in good agreement with spectroscopic data. A simple valence bond explanation has been provided for the qualitative difference between the dipole moment function of HI and those of the smaller hydrogen halides. © 1977.