Mesophyll conductance (g ) is one of the most important components in plant photosynthesis. Tropospheric ozone (O ) and drought impair physiological processes, causing damage to photosynthetic systems. However, the combined effects of O and drought on g are still largely unclear. We investigated leaf gas exchange during mid-summer in three Mediterranean oaks exposed to O (ambient [35.2 nmol mol as daily mean]; 1.4 × ambient) and water treatments (WW [well-watered] and WD [water-deficit]). We also examined if leaf traits (leaf mass per area [LMA], foliar abscisic acid concentration [ABA]) could influence the diffusion of CO inside a leaf. The combination of O and WD significantly decreased net photosynthetic rate (P ) regardless of the species. The reduction of photosynthesis was associated with a decrease in g and stomatal conductance (g ) in evergreen Q. ilex, while the two deciduous oaks (Q. pubescens, Q. robur) also showed a reduction of the maximum rate of carboxylation (V ) and maximum electron transport rate (J ) with decreased diffusive conductance parameters. The reduction of g was correlated with increased [ABA] in the three oaks, whereas there was a negative correlation between g with LMA in Q. pubescens. Interestingly, two deciduous oaks showed a weak or no significant correlation between g and ABA under high O and WD due to impaired stomatal physiological behaviour, indicating that the reduction of P was related to g rather than g . The results suggest that g plays an important role in plant carbon gain under concurrent increases in the severity of drought and O pollution.