An unusual transformation of 2-(5-aryl-2-oxo-3H-pyrrol-3-ylidene)malononitriles occurring through a nucleophilic addition to the C5 atom of the heterocycle is described. The observed process is reversible and characterized by a contrasting color change of the sample from dark violet to almost colorless both in the solid state and solution. The existence of a temperature-controlled equilibrium between the substrate and the reaction product has been established, and the possibility of its directed shift has been shown.