We have investigated N incorporation in Ga(As,N) grown by radio frequency (RF) plasma-assisted molecular beam epitaxy in a wide range of N, flow rate and RF power. Atomic and molecular emission line intensities of nitrogen plasma have been measured by optical spectroscopy. For low N-2, flow rates, the reactive nitrogen species that mostly incorporate are nitrogen atoms whereas for higher flow rates excited molecular nitrogen incorporation seems to prevail. For low N2 flow rates, the N density in the plasma has been related to excited atomic and molecular species emission lines, leading to the determination of the dissociation rate which was found to increase above 0.6 for high RF powers. Finally, impurities related to the N source have been detected in Ga(As,N), especially oxygen, responsible for nonradiative recombination centres, whose atomic emission line has been identified in the plasma emission spectrum. These results are crucial to further control N incorporation in (Ga,In)(As,N) materials and improve crystal quality. (C) 2002 Elsevier Science B.V. All rights reserved.