We performed laboratory experiments on the formation of water and its isotopologues by surface reactions of hydrogen peroxide (H₂O₂) with hydrogen (H) atoms and their deuterated counterparts (D₂O₂, D) at 10–30 K. High-purity H₂O₂ (>95%) was prepared in situ by the codeposition of molecular oxygen and H atoms at relatively high temperatures (45–50 K). We determined that the high-purity H₂O₂ solid reacts with both H and deuterium (D) atoms at 10–30 K despite the large activation barriers (∼2000 K). Moreover, the reaction rate for H atoms is approximately 45 times faster than that for D atoms at 15 K. Thus, the observed large isotope effect indicates that these reactions occurred through quantum tunneling. We propose that the observed HDO/H₂O ratio in molecular clouds might be a good tool for the estimation of the atomic D/H ratio in those environments.