Simplified reaction network illustrating the important reactions in the deuteration of water and other molecules. The left-hand side illustrates the cold gas-phase chemistry, leading to high fractionation of gaseous H₂ D⁺ and atomic D, and ultimately gaseous HDO and D₂O. The right-hand side illustrates how this enhanced D ends up on the ice and leads to enhanced solid HDO and D₂O. The bottom cartoon shows the different ice layers on a grain: the H₂O-dominated layer formed early in the cloud evolution with low HDO/H₂O < 10⁻³, and the CO-dominated layer formed at higher densities which is rich in deuterated water, with D₂O/HDO >>HDO/H₂O (Furuya et al. 2015).