Surface treatment of substrates prior to the adhesive bonding process plays an important role in improving the mechanical properties and durability of bonded joints. In this context, the replacement of hazardous chemicals and pretreatment processes with safer chemicals and more environmentally friendly technologies is essential. The aim of this study was to investigate the fundamental effects of the type of pretreatment and the metal oxide layer on the tensile strength of bonded joints. To prevent reoxidation after removal of the oxide layer, an oxygen-free atmosphere was generated by doping an inert gas atmosphere with the reactive gas silane. The material used for the experimental studies was aluminum alloy AlMg4.5Mn bonded with epoxy-based adhesive Scotch-Weld DP410. Chemical, mechanical and physical pretreatments were investigated with and without metal oxide layer. The results show that removing the oxide layer enables higher tensile strengths. The type of pretreatment is decisive for the final strength.