Ferric oxide admixtures in the concentration range 0.1–10 % in soils, sediments, and clay mineral samples can be conveniently characterized by Vis diffuse reflectance spectroscopy (DRS) in the region of d-d electron transitions close to 500 nm, voltammetry of microparticles, and voltammetry with carbon paste electroactive electrode (CPEE). DRS also detects Fe(III) in the clay mineral matrix equally sensitively. Voltammetry of microparticles is suitable for direct detection of free crystalline and amorphous ferric oxides. The determination limit of total free ferric oxides with CPEE is ~0.01 %. The techniques were tested with six clay mineral samples. XRD was able to detect free FeOOH in only one of them (1.6 %), DRS and CPEE also detected 0.19 % FeOOH in Be-3 (bentonite of Rokle deposit, Bohemia) and 0.04 % in reference montmorillonite Swy-2.