The research presented in this paper focused on the pressure leaching of pyrite and chalcopyrite in their mixture at a low temperature (100 ± 2 °C). The mathematical models of chalcopyrite and pyrite dissolution in their mixture are obtained. According to kinetic analyses, the oxidation process of chalcopyrite and pyrite is limited by intra-diffusion limitations. An elemental sulfur film passivates the surface of chalcopyrite and pyrite particles according to the SEM and EDX mappings. The data show that the oxidation mechanism of chalcopyrite and pyrite in their mixture has changed. The activation energy values of chalcopyrite and pyrite have increased from 51.2 to 59.0 kJ/mol, respectively. The oxidation degree of pyrite in its mixture with chalcopyrite increased significantly from 54.5 to 80.3% within 0-230 min. Copper and iron ions during oxidation were not associated with an increase in the dissolution degree of pyrite with the addition of chalcopyrite. The positive effect of pyrite in its mixture with chalcopyrite on its oxidation degree can be explained by the formation of an electrochemical bond between the minerals. Microphotographs and EDX mapping confirm that the positive effect of the chalcopyrite additive is correlated with a decrease in the formation of elemental sulfur on the pyrite surface. With no formation of conglomerates, the mineral's sulfur content becomes more uniform, confirming the sulfides' interaction with each other.