Oxygen isotopic composition (Δ¹⁸O_LW) of leaf water can help improve our understanding of how anatomy interacts with physiology to influence leaf water transport. Leaf water isotope models of Δ¹⁸O_LW such as the Péclet effect model have been developed to predict Δ¹⁸O_LW, and it incorporates transpiration rate (E) and the mixing length between unenriched xylem water and enriched mesophyll water, which can occur in the mesophyll (L_m) or veins (L_v). Here we used two cell wall composition mutants grown under two light intensities and RH to evaluate the effect of cell wall composition on Δ¹⁸O_LW. In maize (Zea mays), the compromised ultrastructure of the suberin lamellae in the bundle sheath of the ALIPHATIC SUBERIN FERULOYL TRANSFERASE mutant (Zmasft) reduced barriers to apoplastic water movement, resulting in higher E and L_v and, consequently, lower Δ¹⁸O_LW. In cellulose synthase-like F6 (Cslf6) mutants and wildtype of rice (Oryza sativa), the difference in Δ¹⁸O_LW in plants grown under high and low growth light intensity co-varied with their differences in stomatal density. These results show that cell wall composition and stomatal density influence Δ¹⁸O_LW by altering the Péclet effect and that stable isotopes can facilitate the development of a physiologically and anatomically explicit water transport model.