Botanical (coenological) composition, leaf area index (LAI) and ecosystem CO2 exchange (synphysiological) investigations were carried out in the stands of a semi-desert sand grassland in Hungary under present-day and experimentally elevated CO2 concentrations. Present-day results prove the presence of three different types of the Festucetum vaginatae danubiale community in a little investigated area because of the high patchiness of this grassland. The types represent different stages of the secondary succession, and are different in synphysiological terms. The most striking observation is that the main factors, which determine the physiological activity, are the ratio of bare soil and the active LAI, or in other terms, stand architecture which trait also separates three types in coenological respect. The magnitude of CO 2 exchange shows considerable temporal variability depending on the available soil water content. Estimated CO2 flux for the growing season in 2001 (from 01 April to 30 September) was 87.82 g CO 2·m-2, while for the dormant season of the years 2000-2001: -453.87 g CO2·m-2, resulting in a net carbon-loss. The elevated CO2 level increased the cover values of most of the monocotyledon species. Under elevated air CO2 the generalists have disappeared, the rate of natural pioneers decreased, the rate of disturbance tolerant species, weeds and competitors increased. The consequence of the increased dominance of the monocots was detected in the physiological down-regulation of the stand scale photosynthetic CO2 assimilation. When measured at present-day 360 μmol.mol-1 CO 2 concentrations, there was no statistically significant difference between the net ecosystem CO2 assimilation rates of the sand grassland stands growing at present-day and elevated air CO2 level. Thus this grassland can not be considered as a significant carbon sink in the future high CO2 concentration.