The purpose of this work was the comparison of imidacloprid transformation in heterogeneous photocatalysis (TiO2), UV254 (nm) photolysis, UV254 (nm)/VUV185 (nm) photolysis and gamma-radiolysis (also pulse radiolysis) in the presence and absence of dissolved oxygen, and the identification of intermediates formed during the applied treatments. All the methods tested were effective in the degradation of imidacloprid in 10(-4) mol dm(-3) aqueous solution. In most cases the mineralization (TOC) and the disappearance of the starting molecules (detected by HPLC/DAD) occurs parallel. However, when the starting molecules were eliminated in oxygenated solutions the highest decrease in TOC values were only 30-40% in UV/VUV and TiO2/O-2 systems. In radiolysis and UV photolytic systems these values were 10-15%. In the absence of oxygen the mineralization was usually slower than in its presence. Beside the dominant photocatalytic process, a small-scale contribution of direct photolysis was observed in TiO2 containing suspensions, irradiated with 300-400 nm light. Hydroxyl radicals show low reactivity with the N-containing aromatic ring, they attack the heterocyclic 5-member ring and the CH2 bridge between the two rings in direct oxidation or in H-abstraction. In these reactions carbon centred alpha-aminoallcyl radicals form. In the presence of dissolved oxygen these radicals transform to hydroxylated or carbonylated stable products. Scavenging experiments suggest that in TiO2 photocatalytic degradation beside hydroxyl radical based transformation, direct charge transfer has also important role in the degradation. Based on the degradation and mineralization results with imidaloprid, photocatalysis, UV/VUV photolysis and radiolysis are recommended for practical application, economic considerations (E-EO) suggest radiolysis as the method requiring lowest electric energy consumption.