An open chamber for the measurement of soil CO2 efflux has been designed which utilizes an air seal to maintain chamber pressure within 0.004 Pa of atmospheric pressure, eliminating any mass flow and ensuring that atmospheric pressure fluctuations are transferred through to the soil surface. The system was used at the Boreal Ecosystem-Atmosphere Study (BOREAS) southern study area old black spruce site through the 1994 and 1996 field campaigns and was found to be robust and simple to use. Consistency was found between soil CO2 effluxes measured with this system and a closed dynamic chamber system. ¿ 1997 American Geophysical Union