The new alloys of the La–Mg–Ni (Ni⁄(La+Mg)=3–4) system absorb and desorb hydrogen at room temperature, and their hydrogen storage capacities are greater than those of conventional AB5-type alloys. We investigated the crystal structures of the La0.7Mg0.3Ni2.5C00.5 (alloy T1) and the La0.75Mg0.25Ni3.0C00.5 (alloy T2) using ICP, SEM-EDX and XRD. We found that alloy T1 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and PuNi3-type La2Mg(Ni, Co)9 phases, and alloy T2 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and Pr5Co19-type La4Mg(Ni, Co)19 phases. These alloy systems had layered structures and showed polytypism that originated from differences in the stacking patterns of the units, which were composed of several [CaCu5]-type layers and a single [MgZn2]-type layer along the c-axis. The crystal structure of La3Mg(Ni, Co)14 was of a hexagonal 2H-Ce2Ni7-type with a=0.5052(1) nm, and c=2.4245(3) nm. La2Mg(Ni, Co)9 had a trigonal 3R-PuNi3-type structure with a=0.5062(1) nm, and c=2.4500(2) nm. La4Mg(Ni, Co)19 had a hexagonal 2H-Pr5Co19-type structure with a=0.5042(2) nm and c=3.2232(5) nm. In all these structures, the La–La distance of the [CaCu5] layer was 0.38–0.40 nm and that of the [MgZn2] layer was 0.32 nm. We also found that Mg occupied the La site in the [MgZn2] layer. Selective occupation by Mg of the La site in the [MgZn2] layer makes the alloy stable against repeated reaction cycles with hydrogen. The alloy system that forms this material group can be described by the general formula Lan+1MgNi5n+4, where n=0,1,2,3,4,….