A problem of oxide non-uniformity in MOS tunnel structures on n-type Si with a 2-3 nm nominal SiO2 thickness d(n) is investigated. Thickness distribution is described with Gauss law. Concepts of modeling of reverse current-voltage characteristics are formulated for metal-oxide-semiconductor (MOS) tunnel structures, regarding the effect of nonuniformity. The current crowding in relatively thin device sections is shown to result in a noticeable transformation of curves for electron and hole tunnel currents. Introduction of a mean effective thickness d(eff) instead of the nominal one is possible only for the weak-inversion regime. The experimental part of this work includes the examination of samples by TEM and atomic force microscope methods, and their electrical characterization. Measured characteristics have been satisfactorily fitted by curves simulated for a standard thickness deviation of sigma = 0.3 nm.