Abstract Formation region and hydrogen desorption and absorption properties of the perovskite-type structure in Li x Na 1− x MgH 3 with x = 0, 0.17, 0.33, 0.50 and 1.00 were studied in the present work. The experimental results are reasonably explained from the viewpoint of the geometric restrictions of ions that are described by so-called Goldschmidt tolerance factors. In NaMgH 3 ( x = 0), two plateau pressures of about 0.040 and 0.15 MPa were clearly detected by hydrogen pressure–composition ( p – c ) isotherm measurement at 673 K. Moreover, NaMgH 3 can be reversibly formed in 1.0 MPa of hydrogen at 673 K, from the decomposed phase of elemental Na and Mg.