The alpha-proteobacterial genus Acidiphilium consists of several acidophilic species, generally known as a part of the mesophilc microbial flora of leaching biotopes. All of them can grow chemoorganotrophically on carbon sources like sugars and many express additional photosynthetic pigments. Thus far, only Ap. acidophilum is known to be capable of chemolithotrophic growth on elemental sulfur oxidation. The oxidation potential of inorganic sulfur species by the other strictly heterotrophic species has not yet been thoroughly investigated. Here, we demonstrate the unequivocal evidence of inorganic sulfur compound oxidation by strains of Ap. cryptum and other Acidiphilium species. Evolutionary and biochemical aspects of this new feature among the heterotrophic Acidiphilium spp. are discussed. This finding will possibly help to solve the long-standing question about the biochemical nature of elemental sulfur oxidation in mesophilic leaching bacteria.