We report a new contrast mechanism in which scanning tunnelling micrographs of a certain class of molecules resemble chemists' structure formulae. The method is based on adding molecular hydrogen below its condensation temperature to the tunnelling junction of a low-temperature scanning tunnelling microscope. In the presence of hydrogen, the scanning tunnelling microscope contrast can be switched between the conventional mapping of the electronic local density of states and the new geometric imaging by selecting the appropriate bias voltage. Scanning tunnelling spectroscopy suggests that the coupling of the electron tunnelling current to an internal degree of freedom in the tunnelling junction is responsible for the geometric contrast. The new scanning tunnelling hydrogen microscopy (STHM) allows the chemical identification of certain molecular species by their structure.