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This study examined the effects of elevated ozone (O₃) and/or carbon dioxide (CO₂) on the diel allocation of photosynthetically fixed carbon in fully expanded leaves of young (growth stages 4–5) spring wheat (Triticum aestivum L. cv. Hanno). Plants were grown in controlled environment chambers and exposed to two O₃ regimes [‘non‐polluted’ air (CF), < 5 nmol mol⁻¹; ‘polluted’ air, CF + 75 nmol mol⁻¹ 7 h d⁻¹] and two CO₂ treatments (‘ambient’, 354/tmol mol⁻¹; ‘elevated’, 700/μmol mol⁻¹) over a 30 d period. Neutral sugars (predominantly sucrose) were found to be the most abundant form of carbohydrate accumulated by leaves during the day, but significant quantities of starch and high degree of polymerization (d.p.) fructans were also present. Elevated concentrations of O₃ and/or CO₂ were found to have marked effects on diel patterns of export, storage and respiration, whilst the proportions of fixed carbon …