Evaporation from microemulsions was investigated at room temperature in the system water-Aerosol OT-cyclohexanone. The water/cyclohexanone weight ratio was varied and a minimum amount of surfactant was used to obtain a microemulsion in the samples used for the evaporation studies.Evaporation to 70 wt.% of the original weight took place with an approximately constant water/cyclohexanone ratio in the vapor, independent of the original water/cyclohexanone ratio. When the samples approached a water content of a few percent by weight, the cyclohexanone/water ratio was strongly increased in the vapor phase. These changes were assigned to differences in colloidal association structures and strong binding of the water molecules to the surfactant at low water content.