THE scanning tunnelling microscope (STM) is now an established tool for the study of ordered metal and semiconductor surfaces on an atomic scale1,2. Here we demonstrate the ability of the STM to image a large class of organic molecular adsorbates with almost atomic resolution3. From the images it is apparent that not only is the STM resolving the individual molecules, but it is also distinguishing between the different functional groups within the molecules. We propose that the contrast mechanism responsible for the well resolved images is modulation of the local work function of the substrate by the polarizable molecular adsorbates. © 1989 Nature Publishing Group.