This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe-2(xdt)(CO)(2)(PMe3)(4), where xdt is edt (ethanedithiolate, 1), pdt (propane-dithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C-2-symmetric structures (Fe-Fe = 2.616 and 2.625 angstrom, respectively). Low-temperature protonation of 1 afforded exclusively. [mu-H1](+), establishing the non intermediacy of the terminal hydride at ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[mu-H1](+) indicates that the barriers for the two protonation pathways differ by similar to 4 kcal/mol. Low temperature P-31{H-1} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe-2(Hpdt)(CO)(2)(PMe3)(4)](+) ([S-H2](+)) This intermediate converts to [t-H2](+) and [mu-H2](+) by first order and second order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe3 ligands are basal.