In order to see the effect of crystallinity and surface conditions of various oxides and hydrous oxides on the sorption of metal ions, the sorption of tracer concentration (~10" 12 M) of neptunium(V) on naturally-occurring iron minerals (goethite, hematite, magnetite), synthetic iron oxide powders (a-Fe 2 0 3 and Fe 3 0 4 ) and laboratory-prepared hydrous iron oxides (a-FeOOH and amorphous hydrous iron oxide) was studied as a function of pH. The sorption was found to strongly depend on the crystal structure as a-FeOOH > amorphous > a-Fe 2 0 3 , Fe 3 0 4 . Hydrous oxides heat-treated at various temperatures were also examined to see the effect of structural changes. The hydrous oxides retained their original α-FeOOH and amorphous structure below 200°C, while both gradually changed into a-Fe 2 0 3 above 300°C. For the hydrous oxides heat-treated at above 300°C, the sorption was qualitatively correlated with their crystallinities. Intense crushing and grinding of the natural hematite and magnetite resulted in higher sorption of Np(V) than the synthetic powders at lower pH, presumably owing to freshly exposed surface conditions.