Many processes in gas, oil and petroleum industries produce off-gases containing H 2 S gas. H 2 S is highly corrosive in the equipment, and it has adverse impacts if discharged to the environment. The objective of this study is a comprehensive theoretical and experimental investigation of H 2 S reactive absorption process in a NaOH solution as a possible removal method. The reaction product is sodium sulfide, Na 2 S, which is a marketable chemical product. This work focused on the performances of H 2 S reactive absorption with high concentration (25-35% v/v) from off-gas stream into a NaOH solution in a spray or packed absorption columns. The research questions included the following: (1) the kinetic study of the mentioned reaction for the process design, (2) the effects of operating conditions and equipment design parameters on the reaction yield in a spray or packed absorption column in viewpoint of optimization and parametric study of operating variables on the target variables such as sodium sulfide solution production or H 2 S removal efficiency. A kinetic study setup (stirred cell reactor) and a spray or packed absorption column pilot were designed and commissioned. The theoretical approach included utilizing mathematical modeling, response surface methodology (RSM), artificial neural network (ANN), optimization technique of genetic algorithm (GA) and calculator standard method of gas compressibility factor based on detail characterization method (AGA-DCM) for modeling the H 2 S reactive absorption process. The kinetic study of H 2 S reaction into NaOH solution was carried out in a stirred cell reactor (SCR) with considering of the impressments possibility of other gas components which exist in off-gas. The results indicate that the effect of reduction pressure on the process yield which is negligible. Therefore, by using the pressure reduction data from H 2 S chemical absorption into NaOH solution in various temperature and absorption solution concentration levels and different hydrodynamic conditions, the concentration-time data were obtained. These results show that the reaction is controller absorption and is can be considered as a pseudo-first order reaction with respect to H 2 S concentration. The reaction constant ( k 0 ) and the reaction activation energy ( E A ) were calculated 3.06×10 24 m 3 kmol -1 s -1 and 1.288×10 5 kJ kmol -1 , respectively. RSM was used for the experimental study of H 2 S reactive absorption in a spray or packed column. The results show that the spray column performance is better than the packed column for the considered process. Therefore, the optimization and the parametric study of operating variables were done by RSM and GA technique. The highest value of the weight percent of sodium sulfide (15.7±0.2 w/w) was produced at the initial NaOH concentration of 19.3% w/w, scrubbing solution temperature of 40°C and liquid-to-gas volumetric ratio of 24.6×10 -3 . The optimization and the parametric study of effective operating variables on the H 2 S removal efficiency determined that the highest achieved value of H 2 S removal efficiency was 98.7±1.2% at the optimum operating conditions predicted by RSM: initial solution pH of 13.6, initial solution temperature of 42.3 °C and liquid-to-gas volumetric ratio of 22×10 -3 . Additionally, the statistical analyses show that ANN model has higher precision than RSM model, based on the comparison with the experimental data. The performance of a packed absorption column was investigated for H 2 S removal process from the off-gas stream, using NaOH and different oxidants solutions such as sodium hypochlorite and hydrogen peroxide. The optimum operating variables for obtaining the maximum H 2 S removal efficiency were determined by employing GA. A modified gas-side mass transfer coefficient by using a tuning coefficeint was calculated for the reactive absorption. With the modified coefficient, a high agreement was achieved between the model and the experimental data (R 2 =0.97). Keywords: Hydrogen sulfide, Sodium hydroxide solution, Kinetic, Reactive absorption, Stirred cell reactor, Spray column, Packed column, Sodium sulfide, Response surface methodology, Artificial neural network, Genetic algorithm.