The present study evaluates the growth kinetics of boride layers at the material surface on AISI 1045 and M2 steels during the paste boriding process. This surface hardening technique produces on the material two characteristic phases FeB, Fe2B and a transition zone, denominated diffusion zone, in the layer/substrate interface. The thermochemical treatment was done at three different temperatures: 1193, 1223 and 1273 K with two treatment times: 2 and 6 h for the 1045 steel, and 1223, 1253 and 1273 K with the same treatment times for M2 steel, modifying the boron potentials in equilibrium at the substrates surfaces. Using the mass balance equation, and assuming a linear concentration profile at the interfaces, the mobility of boron was determined OD both types of steels. The influence of boron potential, treatment time and temperatures is clearly observed on the growth kinetics of boride layers. (c) 2006 Elsevier B.V All rights reserved.