The stability of ionic liquids with respect to high voltage differences is important for their use in electrochemical applications. Ionic liquids decompose when voltage differences larger than their electrochemical window (4–6 V) are applied. However, little is known about the decomposition mechanism and products. In this work the electrochemical breakdown of the ionic liquids 1,1-butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium tetrafluoroborate on the cathode is predicted using quantum chemical calculations and validated by experiments. The quantum chemical calculations showed to be an excellent method to predict the electrochemical decomposition reactions and products.