A new model for mathematically describing the thickness inhomogeneities of SiO2 layers of MOS structures is presented. This model is compatible with the mechanisms of silicon oxidation and, hence, it can be used to study the influence of the oxidation conditions on the final roughness of the SiO2 layer interfaces. The experimental SiOx interface transition layer has been re-interpreted in terms of the interface micro-roughness and, using the proposed thickness distribution function, the shape of this transition layer is obtained. The experimental thickness extent of this transition layer is well fitted in the presented model with a unity step of the order of the SiO2 cell dimension (≈7 Å). © 1989.