(E)-2-Oxoindolin-3-ylidene ketones can be easily isomerized to their (Z)-isomers by AlCl3 at room temperature in CH2Cl2. The behaviour of the unsaturated dicarbonyl framework in the (Z)-configuration as a bidentate ligand can be the key-step of the isomerization. The limits of a reaction that allows to prepare several yet-unknown products is discussed.