SINGLET molecular oxygen is a very powerful oxidant. Its action is important in a variety of chemical and biological processes(1-4), for examples dye-sensitised photooxidation of lipids, proteins and nucleic acids(4), photodynamic inactivation of viruses(5) and cells(4), phototherapy of cancer(6,7), carcinogenesis(8), haemolysis of erythocytes(9), sensitisation of the human skin(4) and degradation of food(4). The methods used to detect singlet oxygen are unspecific, of low sensitivity or laborious. Photooxidation of 1,3-diphenylisobenzofuran seems to be the most widely used diagnostic test for (1)O(2). However, in the absence of additional control experiments this test does not prove the intermediacy of (1)O(2) (ref. 4) and 1,3-diphenylisobenzofuran has very low solubility and dimerises in aqueous solutions. Lion et al.(10) have proposed a new method to detect (1)O(2) involving the generation of stable nitroxide radicals when (1)O(2) reacts with the sterically hindered amine 2,2,6,6,-tetramethylpiperidin. When using this method to detect (1)O(2) in neutral aqueous solutions, we found no radical production. We report here our investigation of this problem, as it is biologically important to be able to detect (1)O(2) production in such solutions.