Four airplanes measured fluxes of momentum, heat, water vapor, and carbon dioxide in 1994, during the intensive field campaigns of the Boreal Ecosystem-Atmosphere Study (BOREAS): the NOAA/ATDD Long-EZ, the NRC Twin Otter, the University of Wyoming King Air, and the NCAR Electra. This paper presents the results of wing-to-wing formation flights comparing the flux measurements from these airplanes. Comparisons of the spectra of wind components, air temperature, water vapor concentration, and carbon dioxide concentration along with the cospectra of these quantities with vertical velocity showed numerous instructive differences. However, among the three airplanes using established techniques, the fluxes and variances of these quantities generally did not differ by more than one would expect, given the separation of the airplanes. Statistics computed from the Long-EZ data, based on a still evolving application of the Global Positioning System, often differed more strongly. ¿ 1997 American Geophysical Union