The reaction kinetics in nitrogen flowing afterglow with mercury vapor addition was studied by optical emission spectroscopy. The DC flowing post-discharge in pure nitrogen was created in a quartz tube at the total gas pressure of 1000 Pa and discharge power of 130 W. The mercury vapors were added into the afterglow at the distance of 30 cm behind the active discharge. The optical emission spectra were measured along the flow tube. Three nitrogen spectral systems--the first positive, the second positive, and the first negative, and after the mercury vapor addition also the mercury resonance line at 254 nm in the spectrum of the second order were identified. The measurement of the spatial dependence of mercury line intensity showed very slow decay of its intensity and the decay rate did not depend on the mercury concentration. In order to explain this behavior, a kinetic model for the reaction in afterglow was developed. This model showed that the state Hg(6 (3)P1), which is the upper state of mercury UV resonance line at 254 nm, is produced by the excitation transfer from nitrogen N2(A ³Σ(u)⁺) metastables to mercury atoms. However, the N2(A ³Σ(u)⁺) metastables are also produced by the reactions following the N atom recombination, and this limits the decay of N2(A ³Σ(u)⁺) metastable concentration and results in very slow decay of mercury resonance line intensity. It was found that N atoms are the most important particles in this late nitrogen afterglow, their volume recombination starts a chain of reactions which produce excited states of molecular nitrogen. In order to explain the decrease of N atom concentration, it was also necessary to include the surface recombination of N atoms to the model. The surface recombination was considered as a first order reaction and wall recombination probability γ = (1.35 ± 0.04) × 10(-6) was determined from the experimental data. Also sensitivity analysis was applied for the analysis of kinetic model in order to reveal the main control parameters in the model.