Recent work demonstrates reduction of aqueous perchlorate by hydrogen at ambient temperatures and pressures using a novel rhenium−palladium bimetal catalyst immobilized on activated carbon (Re/Pd-AC). This study examines the influence of Re speciation on catalyst activity and stability. Rates of perchlorate reduction are linearly dependent on Re content from 0−6 wt %, but no further increases are observed at higher Re contents. Surface-immobilized Re shows varying stability and speciation both in oxic versus H2-reducing environments and as a function of Re content. In oxic solutions, Re immobilization is dictated by sorption of the Re(VII) precursor, perrhenate (ReO4−), to activated carbon via electrostatic interactions. Under H2-reducing conditions, Re immobilization is significantly improved and leaching is minimized by ReO4− reduction to more reduced species on the catalyst surface. X-ray photoelectron spectroscopy shows two different Re binding energy states under H2-reducing conditions that correspond most closely to Re(V)/Re(IV) and Re(I) reference standards, respectively. The distribution of the two redox states varies with Re content, with the latter predominating at lower Re contents where catalyst activity is more strongly dependent on Re content. Results demonstrate that both lower Re contents and the maintenance of H2-reducing conditions are key elements in stabilizing the active Re surface species that are needed for sustained catalytic perchlorate treatment.