In the context of the ever harsher production conditions encountered in the oil and gas industry, there is a need for improved materials qualification methods for sour (H2S) environments. The classic approach, based on H2S partial pressure, does not adequately characterize the corrosive severity of, in particular, high-pressure environments such as high-pressure high-temperature wells. This paper describes the introduction of non-ideal thermodynamics in the characterization of environmental severity. The focus is on sour exposure and using H2S fugacity or H2S aqueous concentration rather than H2S partial pressure. Experimental work has shown that these approaches are valid, at least for sulfide stress corrosion cracking of steel, and some of the benefits are demonstrated. It is recommended to extend this work to other materials and degradation mechanisms and also to upgrade laboratory test methods and industry standards for this purpose.