Corrosion tests in laboratories to investigate the corrosion rate and to qualify production chemicals are only valid and applicable if the conditions and chemical environment (pressure, temperature, wall shear, pH, CO2 and H2S partial pressures, electrolyte composition) are representative of the field conditions. Representative sulfide concentrations are sometimes achieved by adding a sulfide salt (e.g., Na2S.9H2O) to minimize health and safety risks associated with the direct use of H2S gas. The current study compares the corrosion of carbon steel in nominally comparable sour conditions generated either using H2S gas or sodium sulfide salt. Tests were performed at 90 °C/101.3 kPa and at 120 °C/20 MPa using H2S partial pressures (pH2S) of 0.1 kPa (0.1 mol% H2S at 1.013 bar), 0.2 kPa (0.2 mol% at 1.013 bar) and 40 kPa (0.2 mol% at 200 bar). The corrosion rate was monitored by linear polarization resistance at ambient pressure and by weight loss measurement at higher pressure. Surface morphology of the coupons was assessed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Ambient pressure linear polarization resistance (LPR) measurements with low H2S partial pressures showed comparable corrosion rates obtained using either H2S gas or sodium sulfide. In contrast, in the tests at higher pressure, significantly lower corrosion rates were measured when using H2S gas than with sodium sulfide. Moreover, microscopy showed pitting when sulfide was introduced as the salt, but pitting was absent when the sour environment was generated by H2S gas. Supportive work on other systems has shown that small changes in in situ conditions, particularly at elevated pressure (high H2S partial pressure), have a significant impact on the recorded corrosion rates, and therefore the ability to accurately match the environment within the test cell between generating the in situ sulfide via salt addition versus H2S gas is likely to be the root cause. In summary, therefore at high H2S partial pressure, particularly where the corrosion becomes dominated by sour corrosion, such issues related to the test methodology, and the ability to achieve representative in situ conditions, may lead to disqualification of corrosion inhibitor candidates that otherwise could have achieved the pass criteria were testing performed using H2S gas.