The adsorption of organic molecules on solid substrates is a fundamental step in many important heterogeneous catalytic processes, and is also becoming an increasingly significant aspect of surface modification in microelectronics and sensing technology. The conformation of adsorbed molecules not only influences the outcome of surface-catalysed reactions but also becomes important for recognition processes involved in chemical sensors. The scanning tunnelling microscope (STM) is uniquely able to monitor surface structures at the individual-molecule level(1), and has been shown previously to be capable of distinguishing between different molecular conformations on a surface(2). Here we show that the geometric configuration (cis or trans) of several simple alkenes chemisorbed on the silicon (100) surface can be determined using the STM, through its ability to identify individual methyl groups. Because both the position and the orientation of these groups can be seen, we can determine the absolute configuration (R or S) for each of the chiral centres formed on chemisorption. Thus the STM can probe enantioselective processes at surfaces at the single-molecule level, and may assist in the development of structured chiral surfaces capable of complex recognition tasks.