Four priority pesticides (isoproturon, thifensulfuron methyl, glyphosate and propyzamide) and their main transformation products (TPs; monodesmethyl isoproturon, thifensulfuron acid, aminomethyl phosphonic acid and a ketone metabolite) were investigated for leaching behaviour from sandy clay loam soil contained in polyvinyl chloride pipes under laboratory conditions. Pesticides and their TPs were applied separately to different soil cores at the maximum recommended rates. The soils were irrigated and leachates were collected each week. Analysis of pesticides and TPs were performed via liquid chromatography mass spectrometry. It was found that isoproturon, monodesmethyl isoproturon, glyphosate, and aminomethyl phosphonic acid showed significant leaching behaviour, and were detected in leachates after a lag phase of 3-4 weeks. Propyzamide and thifensulfuron methyl were not detected in leachates nor were their TPs. Aminomethyl phosphonic acid showed a weak leaching tendency by comparison with glyphosate. It is evident that polarity as well as the ionic properties of the pesticides is important for leaching in clay loam soil. Being more polar, TPs should also be considered during risk assessment and monitoring studies of surface and ground waters.