The research of this paper was carried out on the low-temperature (100 ± 2 ◦C) pressure leaching of pyrite and chalcopyrite in their mixture. The resulting mathematical models of chalcopyrite and pyrite dissolution in their mixture, depending on the effect of oxygen pressure, initial concentration of sulfuric acid, concentrations of iron (III) and copper (II) ions and duration are obtained. According to kinetic analysis the oxidation process of chalcopyrite and pyrite proceeds with intra-diffusion limitations. The process is limited by the diffusion of reagents through the solid reaction product layer. According to the SEM and EDX mappings, the surface of chalcopyrite and pyrite particles is passivated by an elemental sulfur film. Analysis of the data allows us to conclude that the oxidation mechanism of chalcopyrite and pyrite in their mixture has changed. This is evidenced by the increase in the activation energy values: from 51.2 up to 59.0 kJ/mol for chalcopyrite and from 50.6 up to 74.6 kJ/mol for pyrite, respectively. The oxidation degree of pyrite in its mixture with chalcopyrite increased significantly from 54.5 up to 80.3% in 0–230 min. The increase in the dissolution degree of pyrite with the addition of chalcopyrite is not associated with an increase in the concentration of copper (II) and iron (III) ions during oxidation, since their effect on the degree of opening of minerals was insignificant, including for the mixture. The positive effect on the oxidation degree of pyrite in its mixture with chalcopyrite can be explained by the formation of an electrochemical bond between the minerals. The positive effect of the chalcopyrite additive is associated with a decrease in elemental sulfur formation on the pyrite surface, which is confirmed by the data of microphotographs and EDX mapping. The elemental sulfur distribution on minerals becomes more uniform with no formation of conglomerates, which also confirms their interaction with each other.