Using the tip of a scanning tunnelling microscope (STM) to mechanically manipulate individual atoms and molecules on a surface is now a well established procedure1,2. Similarly, selective vibrational excitation of adsorbed molecules with an STM tip to induce motion or dissociation has been widely demonstrated3,4. Such experiments are usually performed on weakly bound atoms that need to be stabilized by operating at cryogenic temperatures. Analogous experiments at room temperature5 are more difficult, because they require relatively strongly bound species that are not perturbed by random thermal fluctuations. But manipulation can still be achieved through electronic excitation of the atom or molecule by the electron current6–11 tunnelling between STM tip and surface at relatively high bias voltages10,11, typically 1–5V.Here we use this approach to selectively dissociate chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)-7 x 7 surface. We map out the final destination of the chlorine daughter atoms, finding that their radial and angular distributions depend on the tunnelling current and hence excitation rate. In our system, one tunnelling electron has nominally sufficient energy to induce dissociation, yet the process requires two electrons. We explain these observations by a two-electron mechanism that couples vibrational excitation and dissociative electron attachment steps.