Local density around excited 1-(dimethylamino)naphthalene in supercritical water was measured by peak shift of fluorescence spectrum from 8 to 37 MPa at 380 degrees C. Local density augmentation was evaluated by the excess density, which was defined as the difference between the local density and bulk density. Local density augmentation was observed to be lower than the critical density (rho(c) = 0.32 g cm(-3)). The excess density exhibited a maximum at about 0.11 g cm(-3), which is lower than the critical density. Our results show that excited-state 1-(dimethylamino)naphthalene has Strong interactions with water molecules and that local density augmentation occurs even if the solute life time is very short (ca. ns order). (c) 2006 Elsevier B.V. All rights reserved.