The oxidative chemiluminescence of phenylhydrazines can be applied to the determination of carbonyl compounds by utilising the attenuation of the signal which occurs on conversion to the corresponding phenylhydrazone. Using an assay for 2,4-dinitrophenylhydrazine, off-line derivatisation for a period of 2 h was found to be optimised for a temperature of 30 °C and for a sulfuric acid concentration of 0.05 M. The optimum conditions for 2,4-dinitrophenylhydrazone formation can be understood in terms of a stepwise mechanism of carbinolamine formation via a zwitterion intermediate. The log–log calibration for the combined derivatisation/assay of hexanal in optimised conditions is linear (r = 0.9931, n = 5) from 1 × 10^–6 to 2 × 10^–5M. The limit of detection calculated as above was 1.7 × 10^–7M (1.7 pmol of hexanal). Applications to the oxidative deterioration of linoleic acid and to the effect of lipoxygenase on linoleic acid are described.