Physiological responses of two provenances of European beech (Fagus sylvatica) were studied in seedlings grown at two [CO2] in combination with four temperature treatments. For the local Danish provenance, the average effect of elevated [CO2] during growth was to increase light-saturated net photosynthesis (An) and instantaneous water-use efficiency or transpiration efficiency (ITE). These increases were strongly related to the temperature treatment. Stomatal conductance (gs) was reduced in seedlings in high [CO2], but there was no statistically significant effect of temperature treatment. Stomatal conductance was 13–26% lower at elevated [CO2] and ITE was 89–156% higher, depending on growth temperature. The effects of [CO2] on An were considerably larger than those shown for many other woody species, but similar to those in other studies on European beech. The absolute value of An for a Romanian provenance of beech was 5–18% lower than in the Danish provenance at low [CO2] and 14–26% lower at high [CO2]. There was no statistically significant interaction between the provenances and [CO2], or between provenance and temperature. A model of the response of An to [CO2] at different temperatures gave predictions close to the measured results, except at the lowest temperature treatment where the model over-predicted the effect of elevated [CO2]. This and measurements of An made at a common, low [CO2] indicated a down-regulation of photosynthesis in the lowest temperature treatment at high [CO2]. Root plus soil respiration on a whole-tree basis (Rtr) was increased by elevated [CO2] at all but the lowest temperature, but no effect was seen of [CO2] on root respiration per unit root d. wt. Mean Rtr on any given date was significantly correlated with An, except at the lowest temperature treatment. It is hypothesized that low temperature limited the ability of the roots to use photosynthates resulting in a feedback inhibition of An when elevated [CO2] was combined with low temperature.