Wastewater networks are intrinsically attacked by the production of H2S, a highly reactive gas known for its extremely aggressive nature. This study employs a range of techniques, such as XRD, SEM–EDS, TGA, and Raman spectrometry, to investigate the interaction between low concentrations of H2S averaging 1.3 ppm and various types of Portland cementitious materials (CEM I, CEM II, CEM III, and CEM V). The objective is to discern the chemical alterations contributing to structural deterioration and to provide a mineralogical characterization of the deterioration layers present on these binders. Through a comprehensive examination of the durability of these materials exposed to low concentrations of H2S in a Paris Region sewage system over nearly four years, the findings extend beyond surface-level observations. Despite the absence of visible signs of deterioration, high-precision analytical techniques reveal significant mineralogical transformations within the cementitious matrix. Expansive products such as gypsum, ettringite, and elemental sulfur are identified in mortar samples, underscoring the critical role of precise analysis in comprehending the deterioration process.