The aim of this series of two publications is to characterise important radical reactions of organic molecules in atmospheric water phases, such as cloud, fog, and rain droplets, or on wetted aerosols. In this first study the reactions occurring in a mixed peroxyl radical system generated from the OH radical attack on a model compound, 2-butoxyethanol, in oxygenated aqueous solution are investigated in the absence of trace compounds typically found in such natural waters. Using gamma-radiation emitted from a Co-60 source, We have produced steady-state concentrations of O-2(-)/HO2- and OH radicals comparable to those predicted for daytime conditions in cloud or fog droplets. The reactions of the peroxyl radicals derived from 2-butoxyethanol (C4H9OCH2CH2OH) in oxygenated aqueous solution have been investigated by measuring detailed product distributions resulting from the oxidation process. The observed and quantified products are mainly esters, ketones, aldehydes and hydroperoxides and accounted for 83% of the removal of 2-butoxyethanol (expressed as a carbon balance). A mechanism for the formation of the reaction products is proposed. It includes the OH attack at four carbon centers of the molecule producing carbon centered radicals, the formation of peroxyl radicals by addition of oxygen to these radicals, the decay of the peroxyl radicals in a reaction system of six different peroxyl radicals and superoxide via bimolecular and unimolecular reactions and the reactions of intermediary produced oxyl radicals. (C) 2000 Elsevier Science Ltd. All rights reserved.