  We determine the relative ionization of deuterium and hydrogen in low
metallicity damped Lyman-alpha (DLA) and sub-DLA systems using a detailed suite
of photoionization simulations. We model metal-poor DLAs as clouds of gas in
pressure equilibrium with a host dark matter halo, exposed to the Haardt &
Madau (2012) background radiation of galaxies and quasars at redshift z~3. Our
results indicate that the deuterium ionization correction correlates with the H
I column density and the ratio of successive ion stages of the most commonly
observed metals. The N(N II) / N(N I) column density ratio provides the most
reliable correction factor, being essentially independent of the gas geometry,
H I column density, and the radiation field. We provide a series of convenient
fitting formulae to calculate the deuterium ionization correction based on
observable quantities. The ionization correction typically does not exceed 0.1
per cent for metal-poor DLAs, which is comfortably below the current
measurement precision (2 per cent). However, the deuterium ionization
correction may need to be applied when a larger sample of D/H measurements
becomes available.
