Recently we have shown that carbon coating supported on the surface of oxide nanoparticles is able to stabilize their size and prevent sintering at high temperatures. The carbon shell can play the role of a nanoreactor where phase or chemical transformations of nanoparticles take place. In the present study the carbon nanoreactor approach was used to study the C12A7:eˉ electride synthesis in the argon atmosphere. For these systems the appearance of free electrons registered by electron paramagnetic resonance (EPR) was observed at moderate temperatures (starting from 1250 °C). In the presence of the carbon shell the material maintains relatively high dispersity even at 1450 °C, which exceeds the melting temperature for C12A7. The possibility of increasing substitution of oxygen anions with electrons in C12A7@C systems was examined by increasing calcination temperature from 1200 up to 1450 °C. Highly sensitive EPR method for qualitative and quantitative characterization of these systems was proposed. It was shown that in absence of the carbon coating conduction electrons appear only at temperatures close to the melting point (above 1360–1380 °C). The electride formation inside the carbon shell occurs due to carbothermal reduction of C12A7 nanoparticles encapsulated inside the shell.