Experimental techniques have been developed to determine in situ hydrogen sulfide (H2S) generation as a function of solution alkalinity, SO42- concentration, and surrounding gas during thermal recovery processes. Experimentally, well-designed laboratory tests have been conducted to quantify the in situ generation of H2S in a surface passivation reactor. The reaction gases were analyzed using a gas analyzer, while the solution pH values were measured using a glass electrode. Solution alkalinity restrained the formation of H2S after subtracting the amount of H2S that neutralizes alkali in the solution. H2S production increases with an increase in SO42- concentrations. CO2 when used as the surrounding gas increases H2S production relative to N-2. In comparison to N-2, the presence of CO2 in the gas phase led to the significant increase in H2S generation and the H2S concentration was up to 32.1% at 280 degrees C. It is essential that the effect of solution pH values and SO42- concentrations on the formation water and surrounding gas be taken into account while predicting produced hydrogen sulfide.