Thermal stability experiments were performed at 100 °C of the dye-sensitized solar cell ruthenium dye N719. The experiments were performed as simple test-tube experiments carried out with colloidal solutions of N719-loaded TiO2 particles. The dye degradation was followed by the use of HPLC-coupled electrospray mass spectrometry. The longest half life of N719 of 84 h at 100 °C was obtained in 3-methoxypropionitrile based electrolytes containing 0.5 M 4-TBP, no Li+ ions and 0.25 M . If 4-TBP was removed from the solution the degradation rate of N719 increased up to 3–15 times depending on the concentration. If Li+ ions were added to the electrolyte the degradation increased by a factor of 4–16 times and the thermal degradation product mixture became more complex. It is suggested that an adsorbed layer of 4-TBP on the TiO2 protects the N719 dye against solvent substitution and oxidation processes. A complexation constant of Li+ ions with 4-TBP equal to 9 M−1 was obtained by 13C NMR. The Li+ complexation with 4-TBP is thought to destroy the adsorbed protection layer of 4-TBP on the TiO2 surface. It is concluded that addition of 4-TBP or other N-additives to the DSC electrolyte is important to enhance the dye life time at elevated temperatures. However, addition of Li+ to the electrolyte should be avoided if the wish is to construct DSCs with high thermal stability.