The cathodic breakdown of thin anodic oxide films on aluminium (3-15 nm) was monitored by current transients of potentiostatic pulse experiments, electrochemical impedance spectroscopy and simultaneously recorded optical reflectivity. The time to breakdown t(B) taken from current transients as the point of inflection, varies in a very wide range from 10(-3) to 10(6)s and depends clearly on the film thickness and on the polarization potential, A mechanism of the cathodic breakdown according to the experimental results is discussed.