Mg addition effects on the structure, surface morphology, pressure-concentration-temperature properties, cyclic behavior, and high rate capability of low-Co containing mischmetal (Mm)-Ni-Co-Mn-Al alloys have been investigated. The results indicated that 4 atom % Mg addition significantly improved the cycle life of this AB(5)-type alloy with low Co content. However, Mg addition caused a small decrease in the high rate capability. Pressure composition isotherm results showed that Mg addition led to an increase of plateau pressure and a loss of capacity. Scanning electron microscopy and electron probe microanalysis results showed that Mg addition resulted in some segregation of an Mg-Mn-Al phase. The decrease in the high rate capability could be attributed to the formation of the Mg-Mn-Al phase. Further results showed that the decrease of Mn/Al ratio and alkali treatment could improve the high rate capability of the Mg-added MmNi(5)-based alloys. (C) 2003 The Electrochemical Society.