The reaction kinetics and mechanisms of three neonicotinoid insecticides, imidacloprid (IMD), thiacloprid (THIA) and acetamiprid (ACT), with singlet oxygen and Rose Bengal excited triplet state is reported. The rate constants (5.5 +/- 0.5) x 10(6), (3.9 +/- 1) x 10(7), and (1.3 +/- 1) x 10(6) M(-1) s(-1) were determined for the chemical reaction of singlet oxygen, and the values (4.8 +/- 1) x 107, (1.5 +/- 1) x 10(8), and (3.6 +/- 1) x 10(7) M(-1) s(-1) for the reactions with Rose Bengal triplet state for IMD, THIA, and ACT, respectively. The most important primary degradation product identified was 6-chloronicotinic acid. The results support a reaction mechanism involving a charge transfer reaction between the insecticides and singlet oxygen or Rose Bengal triplet state. The toxicity of the solutions towards Vibrio fischeri seems not to decrease during the early stages of the reactions. Depletion of the insecticides by singlet oxygen and naturally occurring excited states might be important abiotic pathways involved in natural waters. (C) 2010 Elsevier B.V. All rights reserved.