La5.5WO11.25−δ is a proton-conducting oxide that shows high protonic conductivity, sufficient electronic conductivity, and stability in moist CO2 environments. However, the H2 flows achieved to date when using La5.5WO11.25−δ membranes are still below the threshold for practical application in industrial processes. With the aim of improving the H2 flow obtained with this material, La5.5WO11.25−δ was doped in the W position by using Re and Mo; the chosen stoichiometry was La5.5W0.8M0.2O11.25−δ. This work presents the electrochemical characterization of these two compounds under reducing conditions, the H2 separation properties, as well as the influence of the H2 concentration in the feed stream, degree of humidification, and operating temperature. Doping with both Re and Mo enabled the magnitude of H2 permeation to be enhanced, reaching unrivaled values of up to 0.095 mL min−1 cm−2 at 700 °C for a La5.5W0.8Re0.2O11.25−δ membrane (760 μm thick). The spent membranes were investigated by using XRD, SEM, and TEM on focused-ion beam lamellas. Furthermore, the stability in CO2-rich and H2S-containing atmospheres was evaluated, and the compounds were shown to be stable in the atmospheres studied.