The dielec. breakdown of thin (d = 3-4 nm) Al and Ta oxide films was investigated by current voltage plots in metal/insulator/metal systems. Dielec. breakdown field strengths, EDB, of 0.6 GV m-1 were found for both oxide types at room temp. Differences appear in the temp. dependence of EDB. Ta oxide films show an unchanged breakdown behavior for temps. ?420 K while Al oxide films lose already 80% of their EDB value in the same temp. range. Time-resolved investigations of the elec. breakdown revealed intermediate states of both oxide types which were stable for several ms being characterized by an enhanced tunnel current. The breakdown voltage clearly scales with the oxide thickness for both oxide types.