The friction-reducing properties of the Modtc/Zndtp combination were studied using a ultrahigh vacuum (UHV) analytical tribometer. The friction tests were performed on a previously formed tribofilm. Transmission electron microscopy observation shows that this tribofilm is composed of a mixture of zinc phosphate zones containing molybdenum and carbon-rich tunes containing zinc and MoS2 single sheets. After UHV friction, the wear scars of both the sliding surfaces were characterized by in situ analytical tools like Auger electron spectroscopy, scanning Auger spectroscopy, and microspot x-ray photoelectron spectroscopy. Low friction is associated to the transfer of a thin MoS2 film to the pin. Zinc, phosphorous, and oxygen are localized in the wear debris surrounding the pin wear scar. The friction-reducing phenomena is explained on the basis of the hard and soft acids and bases principle.