Stacking faults on basal planes as well as intergrowth structures in La5Ni19 and La2Ni7 have been characterized by high-resolution transmission electron microscopy (HREM) and electron diffraction. Most stacking faults observed in La5Ni19 are of the interblock-layer type in which the faulting in the stacking sequence of block layers occurs without changing the number of unit layers in each block layer. In contrast, most stacking faults observed in La2Ni7 are of the intra-block-layer type in which the faulting occurs in the number of unit layers in block layers, although some stacking faults of the inter-block-layer type are also observed. Intergrowth structures are commonly observed in as-cast alloys with compositions corresponding to La5Ni19 and La2Ni7. The implication of these results for increasing the hydrogen absorption rate of LaNi5-based alloys without significantly affecting hydrogen absorption/desorption properties described in terms of pressure-composition isotherms are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.