Hydrogen adsorption in defective multi-walled carbon nanotubes was investigated. The hydrogen adsorption measurements were conducted using a volumetric approach over a temperature range of 77 to 90 K with pressure below 0.1 MPa. The Dubinin-Radushkevitch equation was used to describe the experimental hydrogen isotherm at different temperatures. The pseudo-vapor pressures (P-s) of hydrogen at 77-90 K were estimated using the Dubinin's approach. Hydrogen adsorption on CNTs in the relative pressure (P/P-s) range of 10(-3)-10(-2), was assigned to microporosity adsorption. The results reveal that the presence of defects, especially in micro-sizes (< 2 nm), on the surface of carbon nanotubes (CNTs) enhance the hydrogen storage capacity due to the strong "pore field". The slope of the linear relation between hydrogen capacity and micropore volume was obtained as 6.0 mass% cm(-3) g at 77 K, when the micropores were fully filled with hydrogen molecules. The results also indicate that the isosteric heat of hydrogen adsorption is significantly affected by defects in the CNT's. (c) 2008 Elsevier B.V. All rights reserved.