This article presents an overview of our recent studies of solvation and unimolecular reactivity in liquids and supercritical fluids. We review our research on rotational diffusion and photoisomerization kinetics of diphenylpolyenes in polar liquids and supercritical fluids. These results provide a qualitative demonstration of the influence of solvent quadrupole-solute dipole interactions on chemical reactivity in supercritical CO2. The CO2 quadrupole moment both increases solvent-solute friction experienced by a dipolar diphenylpolyene solute molecule and diminishes the activation barrier of the isomerization reaction via interaction with the polar transition state. We also review our recent studies of quadrupolar solvation and excited-state excimer formation kinetics of ADMA (1-[9-anthryl]-3-[4-N,N-dimethylaniline] propane). The ADMA studies have yielded quantitative confirmation of the importance of quadrupolar solvent-dipolar solute interactions in solvation and reactivity in supercritical CO2, The results of these studies allow us to measure the contribution of solvent quadrupole-solute dipole interaction to the solvation energy of the excited-state charge transfer form of ADMA. We also discuss recent evidence indicating that the CO2 quadrupole moment influences the kinetics of the charge-transfer reaction.