The photo-semitised oxidation of phenyl linolenate and phenyl arachidonate was studied using four different types of dye sensitiser (methylene blue (MB), erythrosin (EFt), haematoporphyrin (HP) and riboflavin (RF)). The primary product formation was analysed by UV-viaibte $pectroacopy, peroxide value (PV) determinations and a combination of chromatographic techniques. The initially formed monohych'operoxides(MHP) were catalytically hydrogenated to their saturated hydroxy derivatives and then separated and qmmtified by normal phaae high performanceliquid chromatography (HPLC) for the determination of the positional isomer distributioa. The selMu'ated fractions were trans-esterified to their methyl esters and the identity of the resulting 9,10,12,13,15 and 16hydroxyatcarates and the 5,6,8,9,11,12,14 and 15-hydroxyarachidates (eicosanoates) confirmed by gas chromatography-mass spectrogopy analysis (GCMS). In addition, the identity of the 9,10,12 and 13.hydroxystearates were confirmed by specific synthesis. The balance between flee radical (Type I) and singlet oxygen (Type II) mechanisms of photo-oxidation was determined from the relative proportions of the positional isomers. The radical contribution increased with the degree of unsaturation of the substrate and showed small variations with the nature of the sansitiser.