Carbon dioxide (CO2) is the most important greenhouse gas, predicted to increase globally from currently 386 to 550 μmol mol–1 by 2050 and cause significant stimulation to plant growth. Consequently, in 2007 and 2008, Australian grains free-air carbon dioxide enrichment (AGFACE) facilities were established at Horsham (36°45′07″S lat., 142°06′52″E long., 127 m elevation) and Walpeup (35°07′20″S lat., 142°00′18″E long., 103 m elevation) in Victoria, Australia to investigate the effects of elevated CO2, water supply and nitrogen fertiliser on crop growth. Understanding the distribution patterns of CO2 inside AGFACE rings is crucial for the interpretation of the crop growth data. In the AGFACE system, the engineering performance goal was set as having at least 80% of the ring area with a CO2 concentration [CO2] at or above 90% of the target concentration at the ring-centre for 80% of the time. The [CO2] was highly variable near the ring-edge where CO2 is emitted and declined non-linearly with the distance downwind and wind speeds. Larger rings maintained the target [CO2] of 550 μmol mol–1 at the ring-centres better than the smaller rings. The spatial variation of [CO2] depended on ring size and the gap between fumigation and canopy heights but not on wind speeds. The variations in the inner 80% of the rings were found to be higher in smaller rings, implying that the larger rings had more areas of relatively uniform [CO2] to conduct experiments.