When sputter-deposited Mo thin films were exposed to sulfate-reducing bacterium Desulfovibrio desulfuricans, dissolved Mo markedly delayed the culture growth and reduced the rate of sulfate reduction. The interaction led to an orange coloration of the culture liquid. X-ray photoelectron spectroscopy of dried culture droplets revealed that Mo dissolution products existed mostly in pentavalent state, and a smaller amount of molybdate and molybdenum disulfide, In contrast, Mo dissolution in uninoculated medium was negligible. Subsequently, different concentrations of molybdate, ranging from 0.1 to 20 mM, were added to the growth medium and it was found that a low concentration of molybdate (1 mM) was able to reduce the culture growth rate and sulfate reduction by forming Mo(V)-S complexes. In order to study the dependence of the degree of interaction upon microbial activity and growth-dependent metabolic products, 1.0 g/L Mo powder was added to (a) the growth medium, (b) a 3-day-old culture, and, (c) the supernatants of 2 h to 5-day-old cultures. Ultraviolet-visible spectroscopy indicated that the Mo(V)-S complexes consisted of a Mo-S compound analogous to a binuclear dioxobridged Mo(V)-cysteine complex (314 nm) and Mo(V)-containing molybdenyl thiocyanate (468 nm). Dissolution of Mo was induced by H2S, a product of the bacterial sulfate reduction, and was further increased probably by sulfur-containing amino groups and proteins.