The detoxication of pesticides after their application is very important from environmental, human health and economical points of view. Thus, the quest of additives which can accelerate degradation of pesticides (on fruit surfaces) is a very actual problem. In this work we have studied the phototransformation of some dicarboximide group fungicides (vinclozolin, procymidone, iprodione) on the model surfaces of silica and cellulose in the presence of the additives: titanium dioxide and ammonium iron (III) citrate which can increase the photodegradation rate for these compounds under sunlight irradiation. All the studied fungicides being exposed to the sunlight irradiation without any additives give the two main products of their decay: 3,5-dichloroaniline and 3,5-dichlorobenzene isocyanat. Product determination and quantitative analyses were performed by LC-MS and GS-MS chromatography. When applying additives, the same products of fungicides phototransformation were detected [1], and the essential acceleration fungicides photodegradation was observed. Furthermore, in the case of titanium dioxide the acceleration of the photoreaction rate was significantly greater (16 times) than that observed for the case of ammonium iron (III) citrate. The mechanism of this acceleration was revealed by quantum-chemical (DFT B3LYP) calculations. Our theoretical data show that the molecules of the above fungicides being considered in their excited electronic states do not show any structural decay. The same absence of structural decompositions was also found in the case of their positively and negatively charged ions. The presence of 3,5-dichloroaniline among the end products allows us to conclude that we have to seek some source of hydrogen atoms (or their precursors: protons). Basing on the fact that the surface of titanium dioxide is wet and on the well known fact that this dioxide can split water under sunlight irradiation we have to take into account the decomposition of water molecules on the above surface. Our calculations have confirmed the above note: water molecules have their ionic dissociation on the TiO2 surface giving free protons in the medium near it. The further calculations demonstrate that the above free protons interact with the molecules of the studied fungicides (considered in their ground electronic state). This interaction leads to the complete decay of their heterocycles and to the formation of 3,5-dichloroaniline and 3,5-dichlorobenzene isocyanat as the main end products, in full accordance with the above experimental data. Thus, finally, we can draw the following main conclusion: the accelerated decay of the molecules of the studied fungicides adsorbed on TiO2 surface is mainly caused by the protons arising by the ionic dissociation of water molecules on this surface. As titanium dioxide (in difference from ammonium iron (III) citrate) has an amphoteric character and splits (by sunlight irradiation) water molecules essentially better than latter does, so its acceleration of the decay rate of the studied fungicides is many times higher than that of ammonium iron (III) citrate.