GaNAs/GaAs(001) growth surfaces were investigated by reflectance difference (RD)/anisotropy spectroscopy. In situ monitoring was carried out in a plasma-assisted molecular-beam epitaxy system during growth. In the growth temperature range of our experiment, the reflection high-energy electron diffraction pattern always showed a (2 X 4) reconstruction pattern, however, we observed three types of RD spectra. It was suggested that the GaNAs growth surface has three phases because RD spectra properly reflect surface electronic states. It is revealed that the phases are independent from nitrogen concentration, but dependent upon growth temperature and AS(4)/Ga flux ratio. In this article, we show the results of the RD observation, present a phase diagram, and discuss possible GaNAs growth surfaces. (c) 2005 American Vacuum Society.