From the many existing grain size refinement techniques, we have investigated the hydrogen heat treatment of the so-called hydrogenation disproportionation desorption recombination (HDDR) process applied to alloys that contain some amounts of elements with a strong affinity for hydrogen, for example Mg, even though the major constituent of these alloys are mostly elements with a weak affinity for hydrogen. We chose Al-(3-7.8)mass%Mg and Cu-lmass%Mg to research the possibility of the occurrence of HDDR phenomena and their effects on the evolution of the microstructure in the Al-Mg and Cu-Mg alloys. Upon the hydrogenation of the Al-Mg alloys, a disproportionation reaction occurred in the formation of MgH2 embedded in the Al matrix phase. In the subsequent hydrogen-desorption of the alloys, MgH2 was decomposed and Mg was resolved into the Al matrix phase, thereby resulting in the original solid solution alloys. This indicates that HDDR phenomena occurred in the Al-Mg alloys. It is found that the grain size of the alloys was of the order of 10 nm after HDDR treatments such as the heat treatment at 350°C under a hydrogen pressure of 7.5 MPa for 72 h, followed by hydrogen desorption at 350°C for 4 h in vacuum. The grain size of the HDDR-treated alloys increased with the decrease in the Mg content. It was found that the Cu-Mg alloys also exhibit HDDR phenomena and that the grain size was refined to the order of 100 nm. The mechanical properties of the treated alloys were also investigated by the nano-indentation method.