The dye-sensitized solar cell dye Z-907, [RuLL′(NCS) 2 ] may loose a thiocyanate ligand at elevated temperatures (80–100°C) by ligand exchange with the solar cell additive 4-tert-butylpyridine (4-TBP) or the electrolyte solvent 3-methoxypropionitrile (3-MPN). The mechanism in homogeneous solution proceeds via three equilibrium reactions Eqs. (1)–(3) with the solvent complex [RuLL′(NCS)(3-MPN)] as an intermediate:[RuLL′(NCS) 2 ]+3-MPN=[RuLL′(NCS)(3-MPN)] + +NCS − (1)[RuLL′(NCS)(3-MPN)] + +4-TBP=[RuLL′(NCS)(4-TBP)] + +3-MPN (2)[RuLL′(NCS) 2 ]+4-TBP=[RuLL′(NCS)(4-TBP)] + +NCS − (3)A similar mechanism describes the heterogeneous substitution reactions of Z-907 attached to the surface of TiO 2 particles with 3-MPN and 4-TBP. All the six homogeneous and heterogeneous rate constants were obtained at 100°C by monitoring the decay of Z-907 and product formation in test-tube experiments by HPLC coupled to UV/vis and electrospray mass spectrometry.A half-lifetime t 1/2 =150h was obtained for the Z-907 dye bound to TiO 2 nanocrystalline particles at 85°C in the presence of 4-TBP and 3-MPN. Dye-sensitized solar cells (DSC) with Z-907 as a sensitizer and application of the so-called “non-robust” electrolytes containing 4-TBP and 3-MPN is therefore not expected to be able to pass a 1000h thermal stress test at 85°C. Addition of thiocyanate to the cell electrolyte may however, eliminate or reduce the problems caused by dye thiocyanate ligand substitution in DSC cells.