The crystallographic structure as well as the thermodynamic and electrochemical properties of the hydrogen absorbing compounds La5Ni19 and La4MgNi19 have been determined. X-ray diffraction and microprobe analysis have shown the presence of a single hexagonal (P6(3)/mmc) phase for the Mg-free binary compound whereas only one composition but two different crystallographic structures, hexagonal (P6(3)/mmc) and rhombohedral (R-3m), are observed for La(4)MgNi19. Each phase can be described as the stacking along the c-axis of n[A(2)B(4)]/m[AB(5)] sub-units with n = 1 and m = 3 and A = La, Mg: B = Ni. Depending on the stacking sequence, either the hexagonal or the rhombohedral phase is obtained. Thermodynamic properties toward hydrogen uptake (capacity and equilibrium pressure) and electrochemical properties (cycling behavior) in alkaline medium have been measured and are compared for these compounds with and without magnesium. It is observed that the Mg-free compound exhibits a much lower reversible hydrogen capacity than the Mg-containing one. However, despite larger capacity, this latter compound remains to be optimized in term of cycle life. (C) 2008 Elsevier Ltd. All rights reserved.