The Relaxed Eddy Accumulation (REA) technique, implemented aboard aircraft, may be used to measure a wide variety of trace gas fluxes at a regional scale. Its principle is rather simple: air is sampled at a constant rate and the flux is calculated by multiplying a constant β (0.58 in field experiment and 0.62 in simulations) by the standard deviation of the vertical velocity and by the difference between the average concentrations of the scalar (trace gas) for updrafts and downdrafts. The storage of the chemical compound in reservoirs allows for trace gas analysis in laboratory, when in situ measurement with fast response and high sensitivity sensors are not available. The REA method was implemented on the Avion de Recherche Atmosph¿rique et de T¿l¿d¿tection aircraft during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) campaign. The main requirement for accurate flux determination is the measurement of the vertical component of wind velocity in real time. A simulation technique was developed to evaluate the performance of an aircraft REA. The influence of the time lag between the vertical velocity (W) measurement and REA control was tested, as well as the offset of W, the threshold, and the filtering imposed on W. Correction factors, used in a deployment of aircraft REA, were deduced from this study. An additional simulation was performed to evaluate the influence of spatial or temporal drifts on the scalar. The simulation showed that the REA method is not more disturbed than the Eddy Correlation method by low frequencies of physical origin, such as topography. The REA method was used during EXPRESSO for the measurement of isoprene fluxes over the wet savanna and the evergreen rain forest. ¿ 2000 American Geophysical Union