We have evaluated the long-term stability of activities and physicochemical properties for nickel nanoparticles supported on Mg-promoted gamma-Al2O3 (MgAl2O4-Al2O3), alpha-Al2O3, and multi-wall carbon nanotubes (CNTs) for supercritical water gasification (SCWG) in a continuous flow reactor at 425 degrees C, 25.2 MPa, using 9 wt.% glycerol as the feed. Ni/CNT achieved the lowest H-2 yields but produced consistent carbon gasification efficiencies (CGEs) similar to 80% during 48 h of operation at steady state, whereas the CGEs from Ni/alpha-Al2O3 and Ni/MgAl2O4-Al2O3 decreased to < 80% when the gasification was extended to 48 h. Characterization results indicated that CNTs showed stable pore structures and Ni/CNT showed the best dissolution resistance of Ni in SCW. Persistent loss of Al into SCW probably contributed to the deactivation of Al2O3-supported Ni catalyst beds. Results also suggest that enhanced interaction of Ni and the supports may be beneficial for reducing dissolution of Ni in SCW.