An open dynamic chamber is a widely used method for soil CO2 efflux measurements. However, any uncontrolled air flow between the inside and outside of the chamber that is created by differences between the inflow and outflow rates can cause errors in soil CO2 efflux measurements. Estimates of these errors are reported here. A system was constructed in which compensation air of known CO2 concentration was introduced into the chamber and the CO2 concentration was measured in the outflow. The flows of air into and out of the chamber were controlled by two separate mass flow controllers, and the pressure difference between the chamber and the ambient air was measured. When the inflow was larger than the outflow, the measured efflux decreased, whereas when the inflow was smaller than the outflow, the efflux increased. With low fluxes this error was statistically significant when the difference between the flow rates was >30%. The difference between flow rates had a larger effect on efflux rates during high efflux than during low efflux. The effect of air mixing inside the chamber on the reliability of the efflux measurement was tested. Without sufficient mixing it was not possible to get stable efflux readings with this type of chamber technique. The open chamber measurements described here were also compared to closed static measurements based on syringe samples taken manually from the chamber. On average the CO2 efflux values measured with closed static technique were 11% lower than those measured with open dynamic method. ¿ 2001 American Geophysical Union