The influence of non-stoichiometry in the voltammetric response of natural chalcocite electrodes in berate and berate ethyl xanthate solution has been investigated. Surface composition was controlled by short circuiting the mineral to a copper electrode for enough time to attain different non-stoichiometric ratios. Voltammograms recorded afterwards reveal different current contributions that can be correlated with a defined composition. Photovoltammetric curves also exhibit composition dependence allowing us thus to correlate semiconductor properties with copper sulphide composition. Non-stoichiometry also affects chemisorption of xanthate as well as the nature of the electro-oxidation products formed at more oxidizing potentials.