The anisotropy properties of fine magnetic particles can be determined by measurements of the initial reversible susceptibility. The effects of magnetostatic interactions on the reversible magnetization in low magnetic fields near zero and saturation remanence are treated in detail with regard to packing fractionpand particle axes orientations. It is shown among other results that for any symmetrical orientation distribution, the reciprocal initial reversible susceptibility decreases linearly withp. Thus, by extrapolation top=0 (the susceptibility without interaction effects) the effective anisotropy constant of the particles can be obtained. From the temperature dependence of the susceptibility it is conjectured that &ggr;‐Fe2O3has ⟨110⟩ easy directions of crystalline anisotropy. The corresponding anisotropy field as a function of temperature between 77° and 400°K was thus determined from susceptibility measurements. For CrO2particles the anisotropy data are already known and are shown to be in good agreement with the present results.