In this paper, via adjusting Mg content in Pr3-xMgxNi9 (x = 0.45-1.2) alloys, a PuNi3-type single-phase Pr2MgNi9 alloy was obtained by powder-sintering method. The solid solubility of Mg in PuNi3-type phase and phase transformation of Pr3-xMgxNi9 (x = 0.45-1.2) alloys at 1173 K sintering temperature, as well as hydrogen storage properties of single-phase RE2MgNi9 (RE = La, Pr, Nd) alloys were subsequently studied. We found that when x increased from 0.45 to 1.0, entrance of Mg into Pr-2 sites of PuNi3-type phase resulted in a phase transformation from Gd2Co7-type to PuNi3-type, reaching the maximum solid solubility at x = 1.0 with a PuNi3-type single phase at 1173 K. As x further increased to 1.2, an MgCu4Sn-type secondary phase formed. That was the phase transformation occurs to increase the super-stacking phase possessing more [A(2)B(4)] slabs with increase of Mg content. Electrochemical results showed that single-phase alloy had good discharge capacity and superior cycling stability. Comparing with PuNi3-type single-phase La2MgNi9 and Nd2MgNi9 alloys, PuNi3-type single-phase Pr2MgNi9 alloy also exhibited preferable cycling stability and high rate dischargeability (HRD), which were 86.3% (at 100 cycles) and 56.7% (at a current density of 1500 mAg(-1)), respectively. (C) 2015 Elsevier Ltd. All rights reserved.