Photo-oxidation of the neem limonoids nimbin and salannin with UV light in the presence of oxygen gives two isomeric lactone products per limonoid, nimbinolide and isonimbinolide, and salanninolide and isosalanninolide, respectively. When compared in insect tests with the important limonoids of neem seeds, azadirachtin, nimbin and salannin, isonimbinolide and isosalanninolide show activity greater than that of nimbin or salannin and in some respects show activity approaching that of azadirachtin. The photo-oxidation products were tested for anti-feedant activity and toxicity against larvae of three species of Lepidoptera, Spodoptera littoralis (Boisd), Spodoptera frugiperda (FE Smith) and Helicoverpa armigera (Hübner) and nymphs of the locusts Schistocerca gregaria (Forskål) and Locusta migratoria (L).