Subject of thermodynamical analysis are the probable boron transport reactions, boron precipitation and boride forming in the reaction space with the use of aluminothermic boriding mixtures and activators such as NaF, NH4F, Na(2)O(.)4BF(3) and (NH4)(2)(.)4BF(3). Aluminothermic boriding of carbon and alloyed steels have been carried out in a powder boriding mixture containing B2O3-20%, Al-7%, Al2O3-72.5%, and (NH4)(2)O(.)4BF(3)-0.5% at a temperature of 1160 K and within a 6 h period. The thickness and the microhardness of the boride layers have been measured and a layer-by-layer chemical spectrum analysis has been made. The proposed structure of an aluminothermic mixture is determined to be suitable for boriding of low-carbon and low-alloyed steels. The boride layers obtained are distinguished for considerable thickness and high microhardness. (C) 2003 Elsevier B.V. All rights reserved.