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We show that ¹⁸O evaporative enrichment of bulk leaf water in grass species can be significantly more enriched than predicted by the Craig–Gordon model, with C₄ grasses considerably more enriched than C₃ grasses. Our results suggest that the unanticipated ¹⁸O leaf water enrichment of grasses is attributable to the progressive evaporative enrichment along parallel veins (a function of both leaf length and interveinal distance), a pattern that does not occur in Dicotyledonous species. We propose that the differential ¹⁸O enrichment of grasses will result in distinct C¹⁸O¹⁶O biospheric signals from grassland and forest ecosystems, allowing for further partitioning of terrestrial carbon fluxes.