Three different mineral sulfide surfaces were studied using an alternating current scanning tunneling microscope (AC-STM). This technique, which is based on an applied alternating current bias voltage, is able to measure both conducting and insulating surfaces with the resolution of an STM. By comparing the three sulfides (CuS, MoS2, and PbS) with H-Si(111), we could examine the third harmonic generation mechanism on semiconductor surfaces. In addition, the tip-induced oxidation of PbS was observed, starting from a clean PbS surface until an insulating oxide film covered the surface. The reaction was observed with real-time imaging, allowing both the nucleation and growth of an insulating film to be observed without loss of resolution due to insufficient DC tunneling current.