Reaction-driven pattern formation at the interface between two thin liquid films is studied experimentally on a vertical substrate when two organic liquids, an acrylate and a secondary amine, respectively, are put in contact. On a clean glass plate, the pure liquids spread without any instability and no film deformation is observed. On the contrary, if the substrate is pre-wetted with the amine followed by deposition of acrylate on the amine, the two liquids react at the edge of the spreading acrylate film. Fractal patterns are observed to develop at the nominal contact line. No instability is obtained in the reverse reactive case of amine deposited on acrylate. A qualitative explanation of the observed reaction-driven patterns is given in terms of solutal Marangoni effects induced by differences in local values of surface tension.