Reconstructions of climate using leaf wax D/H ratios (?Dwax) require accounting for the apparent isotopic fractionation (?app) between plant source water and waxes. There have been conflicting publications on whether plants in the Arctic growing under 24-hour continuous light, fractionate less than temperate and tropical plants. In this study, we examine the effect of diurnal light (DL) versus 24-hour continuous light (CL) on the isotopic composition of leaf n-alkanes and n-acids in greenhouse experiments using two common Arctic plants (Eriophorum vaginatum, or tussock cottongrass and Betula nana, or dwarf birch). For E. vaginatum, the ?Dwax values of various wax homologues were 5–11‰ more positive for CL plants relative to their DL counterparts, whereas for B. nana, CL waxes were 3–24‰ more negative, suggesting that daylight length is not a unifying control on leaf wax D/H ratios of Arctic plants. The ?13Cwax of B. nana was more negative for plants grown in continuous light compared to diurnal light, reflecting lower water-use efficiency, associated with prolonged stomatal opening in the CL treatment. We modeled the impact of increasing stomatal conductance and effective flow path lengths (mimicking variable leaf morphologies) on the isotopic composition of leaf waters (?Dlw) and find that variations in leaf-water enrichment may explain the variable ?Dwax responses seen between E. vaginatum and B. nana. We suggest that between-species differences in the ?Dlw response to light, and differences in the utilization of stored carbohydrates, were important for governing ?Dwax. Our greenhouse results suggest that Arctic plant leaf waxes do not consistently display reduced ?app values as a result of 24-hour day light, providing additional support for field observations.