Summary: Plots of partition coefficient (mineral/matrix) against ionic radius of the elements in seventeen biotites from trachytes and rhyolites of the Euganean Hills do not show differences in the type of diagram between the biotites with tetrahedral sites completely filled by Si and Al, and those in which filling is only possible with the intervention of Fe³⁺ (or Ti⁴⁺) since these sites are incompletely filled by Si and Al. This behaviour seems to exclude, for the latter, the presence of Fe³⁺ (or Ti⁴⁺ in the tetrahedral site. Similar indications are given by structural analysis. The possibility of existence of [(OH)₄]⁴⁻ groups or, more probably, vacancies in these sites is therefore suggested. The consequences that these results would have on the possibility of correct calculation of the crystallo-chemical formula are discussed.