Abstract: For studying the deacidification mechanism and technics of fumed silica,thermogravimetric analysis was applied to different specific surface area samples without deacidification and their pH values were determinated.It has acquired the content relationship between H2O and HCl on nano-sized fumed silica after treated 30 min in fluidized bed at certain temperature from room-temperature to 400 ℃.Results showed that the adsorption content of H2O was far larger than HCl on samples which have not been deacidified.HCl and physical adsorption H2O were almost completely desorbed at 250 ℃,then chemical adsorption H2O was started to desorbe.In general,HCl and most H2O adsorbed on nano-sized fumed silica were in the form of physical adsorption and were easily to be desorbed.At 250 ℃ to 400 ℃,little change of pH was detected due to the differences of surface structure of silica nano particles at certain temperature.: