A flux measuring system has been developed based on relaxed eddy accumulation (REA, or conditional sampling) methodology. The system uses sampling tubes packed with solid substrates to collect air samples for determining the average concentration of compounds of interest. It has been designed for measuring fluxes of various trace gas species by using appropriate solid substrates in the tubes. The chief advantages to this design are the following: (1) the placement of pump and flow control apparatus are located downstream of the sampling tubes to eliminate sample heating and reduce contamination of the sample stream; (2) the addition of a zero gas channel available to the sampling tubes improves the flow stability without diluting the sample; (3) the design permits the use of multiple sample tube magazines to allow sequential runs without interruption; and (4) the use of compact, easily transportable sampling tubes permit analyses to be conducted at an off-site laboratory where analytical conditions are better controlled. The REA system was tested by measuring water vapor flux under field conditions and the resulting measurements are compared with those made using a Bowen ratio system. The two methods gave similar results with fluxes values that agreed within 12%, although the measurements from the REA system were generally higher than those from the Bowen ratio method. ¿ American Geophysical Union 1995