The electrochemical oxidation and reduction of the surface of a natural tennantite has been investigated in 0.1 M HCl solution using cyclic voltammetry, chronoamperometry and a rotating ring-disc electrode. Subsequent surface analyses by in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS), together with aqueous phase analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) were used to aid in the interpretation of the electrochemical behaviour of this complex system, coupled with Nernst equation predictions of the surface reactions. The XPS results indicated that elemental sulfur, arsenic(III) and arsenic(V) oxides were formed on the surface at electrode potentials of 0 and 0.6 V (SCE), respectively. The Raman spectra of tennantite showed that elemental sulfur was detected on the electrode surface only at potentials 1 V (SCE), at least for the charge densities passed after 500 s. At potentials <-0.4 V (SCE), the intensities of the characteristic Raman spectra for sulfur decreased with decreasing potentials, due to the dissolution of sulfur as H2S (and possibly arsenic as AsH3), leaving Cu3As or elemental copper and iron accumulating on the surface.