An extended volumetric method, combined with quadrupole mass spectroscopy (QMS), is proposed. This method enables us to distinguish and simultaneously quantify hydride (H-) ions and electrons (e-) incorporated in cages of 12CaO∙Al₂O₃ (C12A7), which is accomplished upon annealing with CaH₂. When a sample is dissolved in a deuterium chloride soln., most of the H- ions and electrons released from cages react to form HD and D₂ mols., resp. These isotope-labeled mols. are then detected by QMS. We have used this method to follow the concns. of H- ions and electrons in C12A7 treated CaH₂ over the thermal treatment time. We found that during the initial seven days of treatment, both concns. increased. Thereafter, the electron concn. begins to decrease, while the H- ion concn. continues to increase towards the theor. max. This diverging behavior is due to differences in their diffusion ratios and thermodn. stabilities. [on SciFinder(R)]