The kinetics of the thiocyanate substitution of the solar cell sensitizer [Ru(Hdcbpy)B2B(NCS)B2B]P2-P, 2 (n-CB4BHB9B)B4BNP+)P, (HB2Bdcbpy = L = 2,2´-bipyridine-4,4´-dicarboxylic acid), known as N719, by acetonitrile, 3-methoxypropionitrile, and 4-tert-butylpyridine (4-TBP) have been determined in both homogenous solutions and colloidal mixtures of N719-dyed TiO2 nanocrystalline particles. Thiocyanate ligand substitution by the solvents (S) acetonitrile or 3-methoxypropionitrile in homogeneous solutions occurs at elevated temperatures (80-110°C) by means of a simple slow pseudo-first-order reaction leading to the formation of the product [RuL2(NCS)(S)]+ with a half life time t1/2 ~ 2000 h of N719 at 80 ºC. If tert-butylpyridine (0.5 M) is added, the end product instead becomes [RuL2(NCS)(4-TBP)]+ with a t1/2 ~1000 h. When N719 is bound to TiO2 particles, the reactions with S and 4-TBP give the same products as occur in the homogenous solutions; however, the reactions are approximately 10 times faster. For the reaction of a colloidal mixture of N719-dyed TiO2 particles in acetonitrile containing 0.5 M 4-TBP, a t1/2(het) of 120 h was calculated at 85°C. The N719-based DSSC cells with acetonitrile and 4-TBP as solvent and additive are therefore not expected to be able to pass a 1000-h thermal stress test in the dark at 85°C due to thermal degradation of the N719 dye. Adding guanidine thiocyanate to the colloidal solutions, however, decreased the rate of [RuL2(NCS)(4-TBP)]+ formation by a factor of 2-10; it thus may be used as an additive to prevent the thermal degradation of thiocyanate-based ruthenium complexes in DSSC solar cells.