Single-wall carbon nanohorn aggregates were characterized by N-2 adsorption at 77 K and 'particle density' measurement using the high pressure He buoyancy method. The single-wall carbon nanohorn aggregate had micropores and its pore volume was 0.11 ml g(-1). The particle density was 1.25 g ml(-1). The particle density was not equal to the solid density of graphite (2.27 g ml(-1)), but it agreed within 15% with the density of the single-wall carbon nanohorn estimated using previous TEM data. Hence single-wall carbon nanohorns have closed micropores covered with single graphene walls. According to Nz adsorption, single-wall carbon nanohorns have a considerably large surface area (308 m(2) g(-1)) and open microporosity which has been ascribed to the inter-particle aggregate structure, the so-called 'dahlia flower type structure'.