We report pressure-driven liquid-phase isotope separation (dead-end filtration) to enrich D and O-18 in natural water using graphene oxide (G-O) and UV-reduced graphene oxide (UV-rG-O) membranes. The isotope diffusivity (molecular diffusion and adsorption separation) was found to be responsible for isotope separation. Adsorption separation is the dominant mechanism for improvements in D and O-18 enrichment via increased G-O loading that leads to the increased number of adsorption sites (epoxy and hydroxyl groups on G-O), and higher degrees of reduction of G-O that result in the narrowing of the nanochannels which decreases the portion of water molecules experiencing molecular diffusion. The best performing membrane was "UV-rG-O" made by exposing a G-O membrane to 24 h UV irradiation from one side, showing enrichment of D of 0.5% for D/H and O-18 of 0.08% for O-18/O-16 in a single-stage experiment, without contribution from the vapor pressure isotope effect. This work improves the understanding of the mechanisms for graphene-based membrane separation of D and O-18 enriched water. (C) 2021 Elsevier Ltd. All rights reserved.