An unusually Mn-rich ilvaite sample from the Perda Niedda mine in Sardinia, Italy, was studied in order to clarify the Mn2+ distribution among the different structural sites, and to observe the structural response of the mineral upon thermally induced oxidation-dehydrogenation. The crystal structure and the chemical composition of one crystal [a = 13.014(5), b = 8.867(3), c = 5.838(4) Å, β = 90.02(4)°] were investigated. X-ray crystal-structure refinement, performed in the Pnam space group, and electron microprobe analyses yielded the formula (Ca0.98Mn2+0..02)(Fe3+Fe2+)(Mn0.72Fe2+0 .28)(Si2O7)O(OH). Crystal chemical details, compared to structural data from literature, led to the assumption that Mn2+ replaces Fe2+, mainly at the M2 site. Annealing experiments and structure refinements were performed in the temperature range 400–690 °C. No phase transition was observed over the entire temperature range. Oxidation of Fe2+ at the M1 site, with concomitant dehydrogenation, was deduced from examination of the structural adjustments occurring as the temperature was increased. A useful model to evaluate a possible OH− × O2− substitution in ilvaite was obtained.