This paper describes the construction and performance of branch bags and a CO2 control system used to fumigate branches of mature Sitka spruce trees with air enriched in CO2 (700 μmolmol-1). It contains some examples of results obtained using the system over the course of the first two growing seasons. The branch bags have run continuously for 2 years with very few problems. CO2 concentrations were within 20 μmol mol-1 of the target concentration for more than 90% of the time. Temperatures within the bags were slightly higher than ambient (1–2 °C) and this had some effect on phenology. Attenuation of quantum flux density (photosynthetically active radiation) was 10–15%. The branch bag system has enabled investigation into the effects of elevated CO2 on mature tissue without the problems and expense of fumigating whole trees. Growth in elevated CO2 resulted in an increase in starch and a decrease in soluble protein content of needles. Stomatal conductance was higher in elevated CO2 grown needles, and there was some evidence of an increase in photosynthetic capacity.