A direct comparison of treatment uniformity and CO2 use of pure and prediluted free-air CO2 enrichment (FACE) systems was conducted in a forest ecosystem. A vertical release pure CO2 fumigation system was superimposed on an existing prediluted CO2 fumigation system and operated on alternate days. The FACE system using prediluted CO2 fumigation technology exhibited less temporal and spatial variability than the pure CO2 fumigation system. The pure CO2 fumigation system tended to over-fumigate the upwind portions of the plot and used 25% more CO2 than the prediluted CO2 fumigation system. The increased CO2 use by the pure CO2 system was exacerbated at low wind speeds. It is not clear if this phenomenon will also be observed in plots with smaller diameters and low-stature vegetation.