Careful neutron diffraction measurements on deuterated low density amorphous (LDA) ice confirm that at 120 K it can be considered a fully 'annealed' structure, as no significant changes are observed in the amorphous spectra until crystallization occurred over time at 130 K. On this basis, the measurement of structural differences between the hydrogenated and deuterated forms of LDA ice at 120 K, have been carried out using 98 keV electromagnetic radiation diffraction techniques. The maximum observed isotope effect in LDA ice is ~ 3.4% at 40 K when compared to the magnitude of the first peak in the electronic structure factor at Q = 1.70 Å⁻¹. This compares to a maximum effect of ~ 1.6% previously measured in liquid water at room temperature (Tomberli et al 2000 J. Phys.: Condens. Matter. 12 2597). The isotope effect is shown to be similar to a temperature shift in the structure of light LDA ice. However, the existence of a first sharp diffraction peak at Q = 1.0 Å⁻¹ in the isotopic difference function is not reproduced in the temperature difference and suggests that additional longer-range correlations are present in the more ordered deuterated form.