The influence of molecular as well as atomic hydrogen on the interfacial properties of pyrite has been investigated. Molecular hydrogen selectively converts FeS2 surfaces other than the {100} to FeS. Atomic hydrogen generated by electrochemical and chemical treatment does not lead to selective attack of the pyrite surface but significantly improves the photocurrent-to-dark current ratio. This experimental result evidences the role of atomic hydrogen in the crystal lattice of the material in increasing the barrier height at the solid-electrolyte junction and decreasing the concentration of electronic states within the forbidden energy region. Atomic hydrogen is believed to associate itself with sulfur atoms adjacent to sulfur vacancies. It thereby attracts negative charges from the iron and transforms the electronically unfavorable FeS states which produce defect levels in the band gap into more favorable FeSH centers.