Determination of the eddy diffusion profile in the mesosphere using SOIR/VEx observations Arnaud Mahieux, Royal Belgian Institute for Space Aeronomy SOIR was an infrared echelle spectrometer on board the ESA Venus Express mission (2006-2014) (Nevejans et al., 2006). It performed solar occultations of the Venus mesosphere and thermosphere in the 2.2 to 4.4 µm range (Mahieux et al., 2008). It studied both sides of the terminator and the database covers all latitudes. The SOIR very high spectral resolution and spectral sampling resolved the ro-vibrational lines of the atmospheric gases. Vertical profiles were retrieved from the spectra. SOIR was sensitive to the main species absorbing in the upper atmosphere, and amongst them CO2 and CO (Vandaele et al., 2016). Considering the CO2 vertical profiles and the hydrostatic equilibrium, we could calculate temperature profiles (Mahieux et al., 2015). In this work, we present computations of the eddy diffusion profile at the terminator based on mean CO and CO2 number density profiles, and the corresponding mean temperature profile. The results are constrained using a 1D vertical photochemical/diffusion code. We provide an eddy diffusion profile that covers the 80 to 130 km altitude region. The homopause is located at an altitude of 127 km. Mahieux, A., et al., 2008. In-flight performance and calibration of SPICAV/SOIR on-board Venus Express. Applied Optics. 47, 2252-65. Mahieux, A., et al., 2015. Update of the Venus density and temperature profiles at high altitude measured by SOIR on board Venus Express. Planet. Space Sci. 113-114, 309-320. Nevejans, D., et al., 2006. Compact high-resolution space-borne echelle grating spectrometer with AOTF based on order sorting for the infrared domain from 2.2 to 4.3 micrometer. Applied Optics. 45, 5191-5206. Vandaele , A. C., et al., 2016. Contribution from SOIR/VEX to the updated Venus International Reference Atmosphere (VIRA). Adv. Space Res. 57, 443-458.