Physical and chemical properties of TSEFLAR (TM)-treated gibbsite/hydrargillite (centrifugal thermal activation) CTA HG were studied with X-ray analysis, TA, EM, BET and IRS. CTA HG product obtained at the temperature range of 330-580 degrees C and contact time similar to 1 s is the one with a transitional non-equilibrated layered hydroxide structure with partly lost hydroxide groups. The CTA HG structure does not correspond to the produced chemical composition Al2O3(.)(0.4-2)H2O and, therefore, the products reveal a higher reactivity. Such products are already readily hydrated at room temperature in water and aqueous ammonia to form eventually the stable bayerite-like hydroxide structure. (c) 2004 Elsevier B.V. All rights reserved.