A comparative study has been performed on the activity of natural iron-containing catalysts, such as Tagan montmorillonite in the iron form (FeMM), shale, iron-pillared (2,5 mmol Fe/g of MM) montmorillonite in the sodium form (NaMM), and NaMM-supported iron catalysts, in the photocatalytic decomposition reaction of concentrated (to 90%) hydrogen sulfide under UV and solar irradiation. It has been shown that the catalyst based on the natural MM in the iron form exhibits the highest activity. On the basis of electron microscopy and Mossbauer spectroscopy data, it has been assumed that the process is mediated by ultrafine particles of reduced iron forms.