Particle-size fractions (O = mean diameter, 5-2 µm, 2-1 µm, and < 1 µm) from the Ap horizon of an Entic Haplustoll from Argentina were treated with the selective-dissolution techniques ammonium oxalate (OX), dithionite-citrate-bicarbonate (DCB), NaOH, and Na-pyrophosphate (PY), and studied by X-ray powder diffraction (XRD) and ⁵⁷Fe Mössbauer spectroscopy (MS). Quartz, feldspar, smectite, illite and interstratified illite-smectite are the dominant minerals whereas Fe oxides and oxy-hydroxides are present in low concentration but increase as particle size decreases. Poorly crystallized oxides (highly Al-substituted hematite and goethite) amounts are lower, comparable to or slightly higher than the hematite amounts in the O = 5-2 µm, 2-1 µm and < 1 µm fractions respectively. This hematite is well crystallized but presents some degree of Al-substitution. Magnetite/maghemite was identified in the 2-1 µm fraction. Paramagnetic Fe³⁺ and Fe²⁺ are associated to the clay minerals and/or hydroxyl-interlayered 2:1 type material present; part of this Fe³⁺ is located in the hydroxy-interlayers its amount being higher in the smallest fraction. Any possible changes after the PY and NaOH treatments were not detected by MS.