The subtidal sediments of the inner Ría de Vigo (NW Spain) are fine-grained, rich in organic matter, and accumulate at rates of around 3 mm/yr. Redoxomorphic pyrites formation was studied in four sediment cores. Four operationally-defined iron fractions were determined (HCl-extracted, silicate-bound, organic and pyritic), and the dimensions and morphology of pyrites (which mainly occurred as isolated framboids 3–5 μm in diameter) were studied by SEM. Sediment appearance and degree of pyritization (DOP) identified an oxic zone limited to the top few millimetres, a suboxic zone that in most cores was just a few decimetres thick (the exception was a core obtained in an area of strong turbulent currents), and an underlying anoxic zone with DOP values of around 60%. Pyritic iron increased at the expense of the HCl-extracted fraction in approximately the top metre, but no further pyrite formation occurred at greater depths. Pyrite formation in these lower levels was not limited by lack of either HCl-extracted iron or metabolizable organic matter, but apparently by lack of sulphate ion, the diffusion of which to these depths was probably prevented by the fine texture of the sediments. Our findings highlight the interest of studying transitional sedimentary environments in order to explore in detail the geochemical parameters controlling pyrite formation.