The FTIR spectroscopic study of isophorone solvation in various media using the C=O group as the microenvironmental probe shows that in aprotic medium (cyclohexane) at least two forms are present, even at low concentrations : the monomolecular form and the aggregated forms (dimers and larger). In hydroxylated organic media (butanol), only monosolvated forms are present. In aqueous solutions, solvated species with various degrees of aggregation and hydration can be identified. Molecular mechanics studies show that isophorone, which is an ol-enone, has a preference for an aggregated form over the monomolecular form. Further, the hydrogen atoms of a water molecule can take part in a stabilizing interaction with the carbonyl oxygen atoms of two isophorone molecules. The information on the solvation states (derived from FTIR spectroscopy) and on the nature of the supramolecular structures (calculated via molecular mechanics) have been correlated to the results of photodimerization. Finally, we propose a mechanism of photoreaction, based on the supramolecular structures which are flexible and therefore have a modifiable molecular structure and whose geometry is determined by the interactions with the medium.