We have investigated the adsorption of hydrogen of multiwall carbon nanotubes synthesized by the pyrolysis method using a lanthanum nickel hydrogen storage alloy as a catalyst. The mechanism of carbon nanotubes synthesized using a La–Ni alloy catalyst is discussed. In the hydrogen atmosphere and high-temperature process of carbon nanotube preparation, the LaNi5 alloy particle may be changed into an intermetallic compound of Ni and La by the analysis of the x-ray diffraction patterns and energy-dispersive x-ray spectroscopy of the nanotubes samples using a La–Ni alloy as a catalyst. The H2 uptake capacity of the carbon nanotubes using a La–Ni alloy catalyst is about 5 wt % through to the pressure of 10 MPa. Using a La–Ni alloy as a catalyst increases the effect of chemistry adsorption in the H2 adsorption of the La–Ni alloy catalyzed carbon nanotubes. The P–C–T curve has an approximate plateau. The La–Ni alloy catalyzed carbon nanotubes have better activation for H2 uptake and larger hydrogen uptake in comparison to the carbon nanotubes without using a La–Ni alloy catalyst, and the hydrogen uptake increases with increasing hydrogen pressure from 2 MPa to 8 MPa at room temperature. The hydrogen uptake versus time showing a hydrogen uptake rate of carbon nanotubes using the La–Ni alloy catalyst was raised by the heat treatment from 1.0 wt % (heat treatment temperature 100 °C) to 2.3 wt % (heat treatment temperature 700 °C) at the H2 pressure of 5 MPa. © 2003 American Institute of Physics.