IIIV semiconductors containing small amounts of Nitrogen ("dilute nitrides") are very promising material systems for optoelectronic applications. There are many studies about growth characterization of Ga(NAs) in metalorganic vapor phase epitaxy (MOVPE), but very little is known about the nitridation process on the GaAs (0 0 1) surface and the influence of the surface reconstruction on the Nitrogen (N) incorporation. Therefore, we investigated GaAs (0 01) surfaces under different tertiarybutylarsine (TBAs) and 1,1-dimethylhydrazine (UDMHy) ambient conditions in MOVPE. For in situ surface characterization reflectance anisotropy spectroscopy (RAS) was used. Under sufficient TBAs stabilization the surface forms an As rich c(4 x 4) beta surface reconstruction. This changes towards a more Ga rich (2 x 6)/(6 x 6)-like reconstruction if additional N is supplied. Therefore, UDMHy seems to enhance the As desorption from the surface or the surface reconstruction changes due to incorporated N. This conversion occurs rapidly within 5 s when UDMHy is supplied and was observed in a temperature range from 450 degrees C to 600 degrees C. The measured RAS spectra are then compared to the RAS spectra obtained during Ga(NAs) growth. These also exhibit a (2 x 6)/(6 x 6)-like surface reconstruction which seems to be necessary to incorporate a sufficient amount of N into the GaAs crystal.