Gold nanoparticles with NIR absorption are promising for photothermal therapy applications. Different syntheses have been proposed and, among them, that based on the reduction of Au( III) with thiosulfate is important because it yields gold nanotriangles (AuNTs) with strong absorption in the near-infrared region. It has been previously reported by others that the nanoparticle surface is covered by relatively weak adsorbed S species (mostly sulfate), which would render the surface easy to functionalize in order to improve biocompatibility and therapeutic effects. In this work we have used XPS, XANES, UV-vis-NIR spectroscopy, HRTEM and AFM to demonstrate that, in contrast to the previous reports, these AuNTs are covered by strongly adsorbed sulfur species (sulfide and polysulfide). A reaction pathway is proposed to explain the presence of reduced sulfur species and the absence of adsorbed sulfates and thiosulfates. Preliminary cytotoxicity assays show that the S-capped AuNTs do not show a deleterious effect for Au concentrations larger than those needed for in vivo photothermal treatments.