The study was undertaken to produce superfine alumina trihydrate (ATH) from Bayer liquor. The specialty grade hydroxides suitable for nonmetallurgical applications should conform to certain qualities such as fineness, purity, brightness, etc. In the present paper, the effects of a number of parameters, such as precipitation, temperature, amount of seed, surface area of seed, precipitation time, soda content of pregnant liquor, addition of aluminium sulfate (modifier), etc., on the particle size of the precipitated ATH were examined. At higher temperatures, the particle size increases, whereas it is reduced at lower temperatures. The seed surface area and seed quantity are found to have marked effects on the particle size of the precipitated ATH. The higher seed specific surface area and the higher seed quantity favour the production of finer-sized particles. The increased soda content in the pregnant liquor also produces finer-sized particles. The most important parameter that influenced the precipitation process with respect to the particle size is the addition of a modifier namely, aluminium sulfate. The particle size is reduced to similar to1 mum even with the addition of a small quantity of aluminium. sulfate. It is observed that the formation of finer-sized particles is associated with the degree of rate of nucleation, while the level of supersaturation controls the degree of rate of the nucleation. The seed dissolution in excess soda, available in Bayer liquor, enhances this supersaturation level. When aluminium sulfate is added, the alumina available in the modifier and excess dissolution of the seed due to the addition of a modifier further increases the supersaturation. The seed dissolution, thus, may be considered as a major factor that controls the nucleation rate. Therefore, the mechanism for the fine ATH precipitation from Bayer liquor may be modified to follow the steps: (I) seed dissolution, (II) nucleation, (III) agglomeration, and (IV) growth. (C) 2002 Elsevier Science B.V All rights reserved.