Solvent induced singlet (2p3s) Rydberg state relaxation dynamics of diazabicyclooctane (DABCO) are studied in van der Waals clusters generated in a supersonic jet expansion. The excited-state decay is determined by a pump (excitation)/probe (ionization) mass selective technique. Solvents employed for these cluster studies include rare gases, saturated hydrocarbons and fluorocarbons, amines, ethers, methyl sulfide, acetonitrile, and aromatics. The nature of the intermediate states to which the nascent singlet (2p3s) Rydberg state decays is determined based on excited-state lifetimes, cluster dissociation products, and ionization energies. At least three solvent induced relaxation pathways can be identified for the nascent DABCO Rydberg state : (1) intersystem crossing to the triplet (2p3s) Rydberg state, (2) internal conversion to an intermolecular charge-transfer state, and (3) energy transfer to an excited valence state of the solvent molecule. The (2p3s) triplet state origin is determined to lie at 564 cm-1 below the singlet state origin. The DABCO intersystem crossing rate is small but it can be significantly enhanced by the solvent molecule. Intermolecular electron transfer occurs in DABCO/amine, ether, and acetonitrile clusters. The electron-transfer state is determined to lie at least 650 cm-1 below the (2p3s) singlet Rydberg state. Energy transfer occurs in DABCO/aromatic clusters. The (2p3s) singlet Rydberg state dynamics are found to depend upon cluster geometry, cluster size, and cluster vibrational energy. These results suggest that the (2p3s) Rydberg state is reactive and susceptible to environmental perturbation.