Acetone photosensitisation of 2'-deoxyguanosine in deaerated aqueous solution gives 8-(2,3,4-trihydroxybutyl)guanine as a major photoproduct. Its structure and that of its tetraacetate have been determined primarily by high resolution 1H NMR and mass spectrometry; a di-isopropylidene derivative has also been prepared. Mechanistic aspects of this novel photochemical transformation are discussed, particularly in relation to the alkaline cleavage of acetone photosensitised DNA at the sites of guanine bases.