Photosynthetic parameters were studied in Arbutus unedo L. trees growing at either ambient (AC) or elevated EC (mean 465 µmol mol-1) CO2 concentration near a natural CO2 vent in Orciatico, Italy Diurnal courses of net photosynthetic rate (PN), ratio of variable to maximum chlorophyll fluorescence (Fv/Fm), and quantum yield of electron transport through photosystem 2 (Φ2) were measured on sun and shade leaves. The contents of N, C, Ca, K, P, and chlorophyll (Chl) and specific leaf area (SLA) in these leaf categories were also determined. A morning peak and midday depression of PN were found for both AC and EC sun leaves. Long-term EC caused little or no down-acclimation of PN in sum leaves. The estimate of total daily CO2 uptake was lower in AC leaves than in EC leaves. In shade leaves, it reached up to 70 % of the value of sun leaves. The Fv/Fm ratio showed decreasing trend in the morning, reached a minimum at midday (90 % of dawn value), and then increased in the afternoon. The EC had no effect on Fv/Fm either in sun or shade leaves. Plants grown near the CO2 spring had lower Chl content, higher SLA, and higher Ca and K contents than plants grown under AC. and M. Barták, A. Raschi, R. Tognetti.