The structural and electronic properties of the relatively long-lived (20 mu s) triplet state of acetone in aqueous solution have been studied using an effective QM/MM molecular dynamics approach (GLOB model) based on non-periodic boundary conditions and localized basis functions. In particular, the striking consequences of the n -> pi* electron promotion on the acetone structure and on its microsolvation have been analyzed in some detail and compared to the behaviour of the acetone ground-state. Moreover, the phosphorescence emission spectrum calculated on sampled molecular configurations, shows a Stokes shift in good agreement with available experimental data. (C) 2008 Elsevier B.V. All rights reserved.