The theory of Kelvin-Helmholtz instability is employed to analyze the instability phenomena of non-reacting and reacting stratified gas flows injected sonically into a liquid. The effect of the mass transfer at the gas-liquid interface on the instability is investigated. It is shown that the mass transfer affects the pressure perturbation which acts to transfer energy from the gas phase to the liquid layer through its evaporating and condensing behavior, its wave-drag and lift components, against forces due to surface tension and liquid viscosity. The dimensionless wave frequency, amplification, and wavelength at the maximum instability are presented as a function of a dimensionless surface tension/viscous parameter and a blowing parameter due to the interfacial mass transfer. The interfacial evaporation is found to enhance the instability while the interfacial condensation is to reduce the instability. The results provide the theoretical explanation of the reported dynamic and instability behavior found in the reacting jet of a HCI gas submerged in the ammonia aqueous solution. Finally, an application to the prediction of the break-off plume length observed in submerged reacting jets is presented and the results are compared to experimental data of the HCl(g)-NH3(aq) system.