The problem of pesticide residues on fruits is of great importance, especially for winemaking industry. One of the most used fungicides against grey mold (Botrytis cinerea) is iprodione with normal degradation time of 28-30 days. The use of additives accelerates this process. The aim of this work is investigation of titanium dioxide, alimentary additive and well known photosensitizer, as an additive for acceleration of photodecay of iprodione. Under sunlight action iprodione gives two main producst in all solvents and in silica: 3,5-dichlorphenyl isocyanat and 3,5- dichloroaniline independent on the presence of titanium dioxide, but in the case of this additive the above process is significantly accelerated (16 times). Product determination and quantitative analyses were performed by LC-MS and GS-MS chromatography (Fig. 1). After carrying out investigation on liquid and solid state we can say that phototransformation pathway with titanium dioxide is the same as that in natural conditions (in silica and organic solvents). The figure shows the GS-MS traces of iprodione on silica with addition of titanium dioxide: 1 - iprodione on TiO2 surface, 2 – iprodione on TiO2 after 6 hours sunlight irradiated). Our DFT B3LYP calculations allow one to reveal the mechanism of accelerated iprodione decay on TiO2. Calculations show that an iprodione molecule does not have any structural decay at any changes of its electronic state. However, the free protons arising due to ionic splitting of water molecules on TiO2 wet surface under above irradiation conditions catalyze the structural decomposition of iprodione. Structural transformations of iprodione are presented in the Scheme below. One can see that the protonation of one of the nitrogen atoms belonging to the five-membered heterocycle and connected with its benzene ring leads to the break of one of the N-C bonds in this heterocycle and formation of the new five-membered heterocycle. The following deprotonation of the positively charged transition system causes its instantaneous splitting into a molecule of 3,5-dichlorphenyl isocyanate and a new molecule containing the rest part of the precursor molecule. Moreover, our quantum-chemical study of some other dicarboximide groupe fungicides gives us the basis for the conclusion that the above Scheme presents the common mechanism for accelerated fungicides decay on TiO2 surface under the sunlight irradiation.