The surface structures of self-assembled monolayers (SAMs) formed by the adsorption of pentachlorobenzenethiol (PCBT) molecules on Au(111) as a function of solution concentration were examined by means of scanning tunneling microscopy (STM) to understand the effect of concentration on the formation of ordered domains. STM imaging revealed that PCBT SAMs formed in a 0.01 or 1 mM ethanol solution at room temperature for 20 min contained small ordered domains in the range of several to 20 nm2 and disordered phases, while PCBT SAMs formed in a 0.1 mM ethanol solution were composed of long-range ordered domains in the range of 20 to 50 nm2, which can be assigned as a (4 x √3)R45 degrees packing structure. Interestingly, the bright aggregated domains stacked by π-π interactions between PCBT rings were usually observed around boundary regions of ordered domains. In addition, X-ray photoelectron spectroscopy measurements revealed that ordered PCBT SAMs on Au(111) were formed via the chemical interactions between the sulfur atom of PCBT and gold surface. Our results obtained here will be very useful in understanding the formation and structure of PCBT SAMs on gold surfaces.