The surfaces of a pre-Columbian tumbaga pectoral disc have been analyzed principally by x-ray photoelectron spectroscopy (XPS), combined with argon ion etching for depth profiling. Comparative data were obtained using an SEM with an energy-dispersive x-ray analysis system, and on experimental surfaces produced on a ternary alloy. It is concluded that the surface decoration was achieved by depletion gilding and either selective burnishing or burnishing and selective chemical etching. Both methods produced similar results. XPS with ion etching can provide reliable results for depth profiling, although the method is too time consuming for routine analysis. -- AATA