A theoretical model is developed to account for the kinetics of the decrease of the density of recombination active centres at the interface of SiO2 layers with Si(111) substrates as a result of treatment by thermalised hydrogen plasma. The recombination active centres at the SiO2/Si interface are ascribed to be the P-b centres. The model developed takes into account the kinetics of the incorporation of hydrogen atoms into the SiO2 layer from the plasma source and their interaction with both passivated and unpassivated P-b centres. A special attention is given to ultrathin (similar to 2 nm) SiO2 layers. A simplification of the general model to this important case allows one to obtain analytical solution for the density of unpassivated recombination active P-b centres as a function of the flow of hydrogen atoms from plasma, the initial density of P-b centres, and the treatment time. The model applicability is verified comparing the results of calculations with the experimentally measured values of the relative density of recombination active centres at the ultrathin SiO2/Si(111) substrate interface upon passivation by atomic hydrogen from thermalised plasma source. A conformity of the model predictions to the experimental results is demonstrated. (C) 2009 Elsevier B.V. All rights reserved.