Functionalizing noble metal surfaces with (bio-)organic molecules is a vibrant field of research, with key applications in medicine, catalysis and molecular electronics. Control over the molecular self-assembly is essential to create functional devices. Here, we exploit our high-pressure, high-temperature scanning tunneling microscope (STM) to relate the effects of controllable parameters (temperature and pressure) to atomic-scale assembly mechanisms. Using methanethiol self-assembly on Au(111) as a model system, we monitor the formation and assembly of the ubiquitous (CH 3 S) 2 Au "staple" motif into row structures at pressures up to 1 bar. We observe a pressure-induced transition from the usual 1 /3 monolayer (ML) saturation coverage in vacuum to 3 /8 ML at 1 bar, thus providing the first evidence for a pressure gap effect for thiol adsorption. Complementing our experiments, we employed dispersion-corrected density functional theory computations to model the formed surface adlayers, corresponding STM images and the underlying equilibrium thermodynamics.