Steam explosion treatment was carried out on almost pure natural celluloses (wood pulps and purified cotton linter) to obtain alkali-soluble cellulose and to clarify the effect of cellulose resources on the changes in morphology, viscosity-average degree of polymerisation Pv, solubility towards aqueous alkali solution Sa, and supermolecular structure of the celluloses. Soft wood pulp was most effectively treated in view of the changes in morphology and Pv, but cotton linter was found to be resistant to the treatment. The degree of breakdown of intramolecular hydrogen bonding at C3 and C6 (Xam(C3) and Xam(C6), respectively), as determined by solid-state cross-polarisation/magic angle sample spinning (CP/MAS) NMR, had a tendency to increase for soft wood pulp by the steam treatment employed here. Contrary to this, the crystallinity and the average crystal size estimated by X-ray diffraction increased. Solubility of the treated wood pulps towards aqueous alkali solution can be improved to 100% when suitable conditions for the steam explosion are chosen to give Pv less than 400 and Xam(C3) greater than 44% and Xam(C6) greater than 33%. Solid-state NMR analysis may suggest that hydrogen bond formation at the C2 hydroxyl group may first take place in the structural change of cellulose during the steam explosion.