Five samples of tremolite asbestos from Italy and USA were fully characterized by ICP-MS, SEM, EMPA, FT-IR, MS, XRPD, to correlate crystal chemistry with chemical reactivity. Iron topochemistry was investigated in detail, due to the role of Fe in the aetiology of respiratory inflammatory diseases. The Italian tremolite samples have different Fe contents (San Mango>Ala di Stura>Rufeno>Castelluccio Superiore), and the USA sample from Maryland shows a value almost double that of the Italian samples. The bulk Fe3+/Fe-tot ratio was quantified by Mossbauer spectroscopy, and the values obtained range from 6% Fe-tot (San Mango tremolite) to 24% Fe-tot (Mt. Rufeno tremolite). A possible site distribution of Fe was derived from combining chemical, spectroscopic (Mossbauer) and structural (Rietveld refinement) data. For all samples Fe2+ was disordered over M(1), M(2) and M(3) sites, whereas Fe3+ was allocated to M(2). Production of HO degrees radical in the presence of hydrogen peroxide from the Italian samples is a measure of chemical reactivity, with the lowest value observed for the Castelluccio Superiore sample, and the highest value for the San Mango sample. Notably, HO degrees radical production is directly related to the Fe occupancy of the M(1) and M(2) octahedra, which are more exposed on the external surface of the fibers than M(3) octahedra and therefore have higher probability of being involved in surface reactions. (C) 2010 Elsevier B.V. All rights reserved.