The thickness profiles of microdroplets spreading spontaneously on solid surfaces have been studied using spatially resolved ellipsometry. Information on interaction and rheology can be extracted. We present experimental findings illustrating how long or short range interactions may induce a wetting transition in an isotropic liquid. The parameters modified to approach the transition are the silica thickness of a silicon substrate and the atmospheric relative humidity. The first one induces a change in long range interaction and the second one a change of conformation in liquid molecules in contact with the solid. (C) 1998 Elsevier Science B.V. All rights reserved.