We report scanninf Auger microscopy (SAM) and x-ray photoemission spectroscopy (XPS) analysis of a Roman lead pipe fistula, the main emphasis being placed on the comparison between the surface and interface chemical composition of the joint and of the rest of the pipe. We find that the tertiarium, a Pb-Sn alloy the Romans used for soldering the joint, has a beneficial effect in limiting the formation of white lead, 2PbCO₃.Pb(OH)₂. Comparison measurements performed in the alloy and the pure separate metals show that tin markedly limits the ambient surface oxidation of lead. Conversely, tin is much more oxidized in the allot than when analysed alone. Auger point spectra indicate that carbon is the main surface contaminant of the patina of the fistula, and Auger imaging is used to pinpoint, with a spatial resolution of <about>0.5mum, the distribution of this element at the interface of a lead/tertiarium solder, performed according to the procedures described by Plinius. The diagnostic capability of Auger transitions of lead and tin for distinguishing between different phases is discussed on the basis of their chemical shifts and surface sensitivities.