A kinetic mechanism of HfO₂ and ZrO₂ film growth in an ALD reactor is suggested to explain the following experimentally observed features of the process:1) formation of less than one mono layer per cycle; 2) the film growth rate dependence on the degree of surface hydroxylation (that is, on temperature); and 3) the residual chorine concentration dependence on process parameters. The mechanism is based on the results of quantum chemical calculations of surface reactions. The rate constants of elementary reactions have been were calculated in the framework of RRKM theory The calculated rate constants were used as starting data for ALD process simulations within the non-stationary surface plug-flow model. The effect of steric hindrances of the precursor ligands has been analyzed in detail using the kinetic Monte Carlo approach. This effect can be quite significant and lead to the reduction of the surface coverage down to 30%. It also restricts the steady state film growth to about 0.4 layers per ALD cycle, which is in good agreement with the available experimental data for ZrO₂ and HfO₂ film growth. It was also proved that surface dehydroxylation can significantly decrease the film growth rate. Based on the calculated kinetic curves, the lowest H₂O adsorption energy on the oxide surface was estimated (about 27 kcal/mol) below which the desorption of water molecules becomes significant. The experimentally observed dependence of the residual chlorine concentration on the temperature can be explained by steric hindrances from chemisorbed surface groups to the chemisorption of MCl₄ molecules (M = Zr, Hf).