Using ambient-pressure X-ray photoelectron spectroscopy, here we report the real-time monitoring of dynamic surface composition evolution of Cu3Au(100) in response to the imposed environmental stimuli. Segregation of Au to the pristine surface under ultrahigh vacuum annealing leads to the phase separation with pure Au at the surface and alloyed Au in the subsurface. Upon switching to an oxidizing atmosphere, oxygen adsorption drives the surface segregation of Cu along with inward migration of pure Au to the subsurface. Switching to a H2 atmosphere results in oxygen loss from the oxygenated surface, thereby promoting Au surface segregation and reverting the surface to the pristine state with the Au termination. These measurements demonstrated the tunability of the surface composition of the binary alloy by utilizing the interplay between the tendency of segregating a more noble constituent to the surface and the tendency to segregate the more reactive one with the chemical stimuli.