This study evaluates the response of flame-retardant polypropylene composites after long-term exposure to aqueous and chemically aggressive media. Polypropylene (PP), used as a reference material, and PP-based composites containing 5, 10, 15, 20, and 25 wt.% of a phosphorus-based intumescent flame-retardant additive were immersed in water, NaCl solution, H₂SO₄ solution, and NaOH solution for up to 200 days. The material behavior was initially assessed by gravimetric measurements, based on the absorption/degradation degree calculated for early, mid, and late exposure intervals. This allowed the evolution of the material–medium interaction to be tracked not only by individual mass variations but also by average trends over the exposure period. The exposed systems were further investigated by thermogravimetric analysis (TGA/DTG) and dynamic mechanical analysis (DMA) to determine whether contact with the selected media modified the thermal degradation profile or the dynamic mechanical response of the composites. The results showed that the incorporated flame-retardant additive increases the interaction of PP with the liquid media, especially at intermediate and high additive contents. TGA/DTG and DMA confirmed that the thermal and viscoelastic behavior of the PP-based systems was maintained after long-term exposure, indicating that the observed medium-induced interactions did not result in substantial deterioration of their bulk thermo-mechanical properties.