The strategy of Schwack to study the photochemical fate of pesticides using model substances in the first step, has proved to be a good tool for the understanding of fundamental photochemical reactions. Furthermore, reaction possibilities of the parent compound or its degradation products with functional groups, occuring in natural plant surfaces, can be identified. This basic knowledge of photodegradation can help to elucidate the reaction pathways in the more complex natural environment and to characterize and quantify bound, nonextractable pesticide residues.The objective of this work was to understand the basic principles of the photochemistry of the neonicotinoid insecticide imidacloprid, especially on fruit surfaces. For this purpose, the following discrete issues were addressed:. Basic photolysis experiments with dissolved imidacloprid in proved model solvents as methanol, ethanol, 2-propanol, cyclohexane and cyclohexene and with solid imidacloprid on a glass surface . Irradiation experiments with the photoproducts 1-[(6-chloropyridin-3yl)methyl]imidazolidin-2-imine and N-nitrosoimidacloprid . Analysis of the reactivity of 1-[(6-chloropyridin-3-yl)methyl]imidazolidin-2-imine towards epoxides using cyclohexene oxide and 9,10-epoxystearic acid as model substances for cutin acids . Irradiation experiments with imidacloprid on tomato fruit cuticles and tomato fruits both under sunlight simulator and natural sunlight conditions . Development of a strategy for the synthesis of 15 N-labeled imidacloprid and characterization of 15 N-imidacloprid and its synthesis intermediates . Application of 15 N-imidacloprid for irradiation experiments on tomato fruits with subsequent analysis of the surface extract, the tomato pulp and the tomato cuticles by elemental analyzer / isotope ratio mass spectrometry (EA/IRMS)