The nitrogen incorporation behavior in GaAs was investigated in the growth temperature range 500-600°C. It was observed that the temperature-dependence of the nitrogen incorporation exhibits two regimes. At 530°C, the nitrogen content x is a nonlinear function of the gas-phase composition indicating a surface-controlled reaction mechanism. The N composition varies slowly with 500°C < T < 560°C with an activation energy of 0.6 eV. For T < 560°C, N decreases exponentially with Ea= 3.7 eV, interpreted in terms of nitrogen desorption. In light of experimental results, we propose a surface kinetic model based on the competitive adsorption of group V precursors.