Kinetic studies on the flotation of a fine-grained complex lead-zinc ore showed that sphalerite exhibits considerable floatability during the later stages of galena flotation, causing a loss of zinc to lead concentrate. The lead concentrates obtained toward the end of the second lead rougher flotation were examined by X-ray photoelectron spectroscopy (XPS). The results showed that the lead-to-zinc atomic ratios on the surface are significantly higher than in the bulk, suggesting that the flotation of sphalerite is caused by the activation of the mineral by the lead ions present in the flotation pulp. Further evidence of the activation mechanism was given by examining the surfaces of the monominerallic sphalerite specimens that had been immersed in the flotation pulp and other solutions containing lead ions; XPS analysis of the specimens showed significant amounts of lead on the surface. When the specimen was contacted by ore pulp (or simulated plant water) and then with a xanthate solution, the Fourier Transform Infrared (FTIR) spectrum of the specimen showed the presence of lead xanthate and dixanthogen. The proposed activation mechanism is discussed in view of the thermodynamic data available in the literature.