Background and aims: ABA plays an important role in modulating stomatal response to drought and elevated atmospheric CO₂ (e [CO₂]). This study aimed to investigate the effect of e[CO₂] on the response of leaf gas exchange and plant water relations of barley and tomato plants with different endogenous ABA levels to progressive soil drying. Methods: Barley and tomato plants were grown in ambient (a[CO₂], 400 ppm) and e[CO₂] (800 ppm) and subjected to progressive drought stress. Wild type (WT) genotypes (Steptoe barley and AC tomato) and their ABA-deficient mutants (Az34 barley and flacca) were examined. Results: e[CO₂] sensitized the photosynthetic decline with soil drying. Soil-drying induced stomatal closure was affected by [CO₂] in WT genotypes, where e[CO₂] sensitized stomatal closure in barley but retarded it in tomato, whereas such effects were absent in mutants. Compared to a[CO₂], e[CO₂] maintained leaf water potential and improved turgor pressure except in the flacca mutant. For the WT genotypes, the stomata became less sensitive to an increase in leaf ABA concentration ([ABA]_leaf) under e[CO₂] than a[CO₂]; while for both mutants, the stomata was predominately controlled by leaf turgor and not an increase in [ABA]_leaf during soil drying. Conclusion: Endogenous ABA level played an important role in modulating the effect of e[CO₂] on stomatal response to soil drying. These findings improve our understanding of the mechanisms of stomatal control in monocot and dicot species responding to a future drier and CO₂-enriched environment.