The photostability of the sensitizer dye [Ru(dcbpyH)2(NCS)2] (Bu4N)2 (referred to as N719) was investigated in a simple model system instead of a complete nanocrystaline dye sensitized titanium dioxide solar cells (nc-DSSC). The applied model system consisted of N719 dyed titanium dioxide nanoparticles, suspended in a cuvette containing acetonitrile as the solvent which were illuminated with 532/525 nm monochromatic light of different light intensity. Under these experimental conditions and in the absence of any redox couple as a regenerative electron donor, the number of mole of photodegradation products: [RuL2(CN)2], [RuL2(NCS)(CN)] and [RuL2(NCS)(ACN)] increased almost linearly as a function of increasing mole of absorbed photons. The total quantum yields of these degradation products, Φdeg showed light intensity dependence, being higher at low light intensities and smaller at higher intensities. This light intensity dependency of the quantum yield was attributed to the back electron transfer reaction rate between the titanium dioxide conduction band electrons and the oxidized dye cation. Photoinduced absorption spectroscopy (PIA) was used to measure the back electron transfer reaction rate, kback at the same light intensities as used in the model experiments. The PIA measurements showed that kback increased with increasing light intensities. By applying the equation kdeg = Φdeg × kback to the experimentally obtained total quantum yields and back electron transfer rates it was possible to calculate an average value for the oxidative degradation rate of the N719 dye attached to the TiO2 nanoparticles, kdeg = 4 × 10–2 s–1 . Heating and illumination of the model system in the presence of 4-tert-butylpyridine (4-TBP), the commonly used additive in nc-DSSC, revealed the possible involvement of the dye in a substitution reaction with 4-TBP with the formation of the substitution product [Ru(dcbpyH)2(NCS)(4-TBP)] –, Bu4N+. It was also found that the amount of the additive substituted product formed under illumination increases upon adsorption of the dye on TiO2 nanoparticles. Quantum yield measurements of the substitution product under different experimental conditions directed us to conclude that the main part of the substitution product is formed through the oxidized state of the dye, S+. The quantum yield of the substitution reaction initiated from the dye oxidized state was φ+ sub (TiO2) = 7.1 × 10–5 . Based on this value the rate of the hotosubstitution reaction was calculated to be k+ sub(TiO2) = 0.07 M–1s–1. The influence of the photosubstitution reaction on the N719 dye lifetime was discussed based on the k+ sub(TiO2 ) value and the literature values of the dye regeneration reaction, kreg between the oxidized dye, S+ and iodide.