The light stability of 0,0-diethyl-0-(4-ethylthiophenyl)phosphorothioate, a parent structure of a new class of fibre-reactive organophosphorus insectproofing agents for use on wool textiles was extensively examined. The rate of degradation of 0,0-diethyl-0-(4-ethylthiophenyl)phosphoro-thioate in polar and non-polar solution and on wool upon irradiation by simulated sunlight was investigated using high performance liquid chromatography.. The major photodegradation products in each case were correlated with the HPLC retention times of synthetically prepared compounds. The main product formed was the sulphoxide, 0,0-diethyl-O-(4-ethylsulphinylphenyl)phosphorothioate, whose insecticidal activity against the major textile pests was shown to be similar to that of the parent compound. In polar solution a polar product which could not be identified was formed. Both 4-ethylsulphinylphenol and 4-ethyIsulphony1-phenol were found on wool but not in solution. The effect of various ultraviolet stabilizers on the rate of photodegradation of 0,0-diethyl-0-(4-ethylthiophenyl)phosphorothioate was also examined. Ultraviolet absorbers of the 2-hydroxybenzophenone and 2-hydroxybenzotriazole classes conferred the best protection in each case. However, on wool typical wool dyes applied at conventional levels were also effective.