An identification of the nature and an estimation of the particle size distribution of the iron deposits on thalassemic heart tissue is carried out by variable temperature Mössbauer spectroscopy. Comparison of Mössbauer spectra obtained for the thalassemic heart tissue (I) with those of normal heart tissue (II) and of horse spleen ferritin (III) identifies the iron deposits to be small, superparamagnetic particles of ferritin and/or hemosiderin, two closely related iron storage proteins containing an iron core of (FeOOH)8(FeO · OPO3H2). The dependence of the superparamagnetic relaxation time, τ2, of magnetically ordered fine particles on their volume V via the magnetic anisotropy constant K of the material and the condition τ > τL, the Larmor precession time of the nuclear magnetic moment of 57Fe about an effective magnetic field, for observation of hyperfine structure are used in analyzing the Mössbauer data to yield the particle size distribution. Particle diameters are estimated to be 74 ± 12 Å.