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Total deuterium (D) retention from the bulk material of reduced-activation ferritic-martensitic (RAFM) steel Eurofer’97 (EU’97) and commercial ferritic-martensitic grade P92 was traced experimentally by means of thermal desorption spectroscopy (TDS) and linked to the role of krypton (Kr) and argon (Ar) seeding during high-fluence plasma exposure. The influence of impurity seeding on the steel microstructure was determined using scanning electron microscopy (SEM) and extended with focused ion beam (FIB) cross-sectioning. D capture at depths in the µm range was measured by nuclear reaction analysis (NRA). Plasma exposure of the steel samples occurred at 470 K with ion energy of 30–40 eV in the linear plasma device PSI-2 with up to 10% simultaneous impurity admixture in plasma. D inventory achieved values in the 1020 D/m2 range after plasma exposure with high fluences of up to 1 × 1026 D+/m2. In pure and mixed plasmas, the majority of D was trapped in the steel bulk. The Kr and Ar seeding of D plasma resulted in the population of multiple already existing D trapping sites. A similarity of D desorption spectra suggests that D trapping in P92 follows the same mechanism as in EU’97. Kr and Ar seeding mostly contributed to the surface sputtering of the steel samples, yielding less D retention due to the material loss. Keywords: Eurofer, RAFM, Ferritic-martensitic, Impurity seeding, Retention, Desorption