The degradation of atrazine and four organophosphorus pesticides (chlorpyriphos, fenamiphos, methidathion and methyl-parathion) in oranges was studied. Oranges were immersed in a Milli-Q water solution spiked with 10 mg litre-1 of each pesticide for one day, allowing their adsorption on the orange peel. Then, the oranges were rinsed with Milli-Q water and left outdoors to expose them to natural ambient conditions for two weeks. In parallel, an aqueous solution containing 1 mg litre-1 of each pesticide was placed in a Pyrex flask, which was tightly closed, and exposed to the same ambient conditions. Both samples (orange peel and Milli-Q water) were analyzed periodically by gas chromatography coupled to a nitrogen-phosphorus detector. The pesticide degradation in both samples could be described using a first-order degradation curve. Half-lives varied from 14.5 to more than 30 days in aqueous solution and from 2.3 to 4.1 days in oranges for organophosphorus pesticides, while those for atrazine were 3.1 days and 14.2 days, respectively. The presence of some organophosphorus degradation products in water samples after storage under the above conditions was confirmed by gas chromatography-mass spectrometry.