(From doi:10.1103/PhysRevLett.98.116102): A scanning tunneling microscope operating at 5 K is used to induce the isomerization of single chloronitrobenzene molecules on Cu(111) and verify the reaction. The threshold voltage of (227ü7) mV for this reaction is explained based on electron-induced vibrational heating. We propose that the isomerization is initiated by simultaneous excitation of two vibrational molecular modes via inelastically tunneling electrons. This excitation results in a shift of the distribution probability of chlorine and hydrogen positions, which facilitates their mutual exchange.