A 1986 programme to measure water vapour and droplet fluxes from sea to air at moderate to high wind speeds. HEXOS was the first comprehensive open ocean air–sea flux field project to emphasise surface exchange processes. It was conducted on the Dutch Noordwijk platform, and featured the largest range of windspeeds over which momentum, sensible heat, and latent heat fluxes had been measured. It provided the physical basis for linking aerosol, sea spray, and sensible and latent heat fluxes under a common framework.