A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope (13C/12C and 18O/16O) ratios of updraft and downdraft air and thus the net fluxes of 13C16O2 and 12C18O16O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earth's boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio (<CO2>) fluctuations or those of another scalar are also used. Simulations using 10-Hz data show that a wind-based/scalar-based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft <CO2> differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down <CO2> differences with HREA of 8--10 ppm are possible, compared with 3--5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach. ¿ 1999 American Geophysical Union