Is the D/H Ratio in the Comet Coma Equal to the D/H Ratio in the Comet Nucleus?

We present a simple, semianalytic model of the vaporization of H₂O and HDO ice from a comet nucleus. We use this model to show that the flux of HDO relative to H₂O can be much higher, at times, than would be expected from the D/H ratio in the nuclear ice itself. This effect varies with position in the comet's orbit. It is negligible sufficiently near the Sun but could lead to erroneous interpretations of the primordial D/H ratio in cometary ice if measurements are made in other parts of the cometary orbit.