Molten carbonate fuel cells (MCFCs) offer several advantages in energy applications, but their lifetime is limited especially by electrolyte depletion. In this study, a novel in situ method for replenishing the electrolyte in MCFCs by injecting gaseous electrolyte precursors (EP) that form liquid electrolyte through reactions with CO2/O2 gases or CO32- ions is introduced. The selection of EP species (LiI, NaI, and KOH) are guided by thermodynamic considerations including high vapour pressures and conversion rates to Li/K or Li/Na carbonates. The proposed approach addresses two electrolytic depletion stages: consumption of the electrolyte in the electrode (Stage 1) and in the matrix pores (Stage 2). Atmospheric-pressure electrochemical vapour deposition (AP-EVD) and atmospheric-pressure chemical vapour deposition (AP-CVD) are demonstrated to replenish electrolyte in the electrode and matrix pores, respectively. The applied techniques sustained a cell with a low (Li/Na)2CO3 electrolyte level for long-term operation (>1,000 h) while maintaining stable performance (>0.8 V at 150 mA cm−2). The method also returned an inactive cell to its normal operating conditions (OCV: 1.054 V, RΩ: 0.61 Ω cm2, (Li/K)2CO3 electrolyte). These results promise enhanced durability of MCFCs and render them much more viable for use in energy applications.