Experiments have been made to determine the behavior of sodium aerosol under various atmospheric conditions, with the view to establishing an air monitoring technique to be used in sodium-cooled fast reactors. Changes in both size and form of sodium aerosol particles released into the air were studied. At levels of humidity below 40%, the particles were observed to be in solid state, constituted of sodium oxides and sodium hydroxide. When sodium aerosol particles are exposed for more than 1 min in an atmosphere containing more than 60% humidity, they are mostly converted into droplets of sodium hydroxide. The size of the droplets increases with humidity, the diameter becoming at 95% humidity about 1.4 times that at 40% (2. 7 times in volume). The final state of sodium aerosol suspended in air with a humidity exceeding 90% is in the form of droplets of sodium carbonate. When the CO2 concentration in the air is as high as 1.0v/o, the droplets solidify even at a humidity of 70%. The droplets can be made to solidify into grains by dehumidification through air heating. The size of the solid particles thus produced is nearly the same as that of the solid particles observed originally at low humidity, regardless of the level of humidity before heating.