Mass-resolved excitation spectra of the (2p3s) <-- (2p)2 Rydberg transition of 1,4-diazabicyclo[2.2.2]octane (DABCO) clustered with polar and nonpolar solvents are reported. The DABCO/solvent clusters are generated in a supersonic expansion. The solvents employed in this study have diverse electronic and dipolar properties and include amines, ethers, aromatics, acetonitrile, a thioether, cyclohexanes, ethylene, N2, and Kr. The spectra can be analyzed in terms of cluster transition origins, van der Waals vibrational modes, and internal DABCO vibrational modes. Potential energy calculations are performed for the cluster ground state using atom-atom potentials. The cluster transition origin shift from that of the isolated DABCO bare molecule is used to characterize the DABCO/solvent interaction in the (2p3s) Rydberg excited state. Blue-shifted transition origins are observed for DABCO/Kr, N2, ethylene, cyclohexane, and methylcyclohexane clusters; these shifts can be interpreted as arising from strong repulsive interactions in the excited Rydberg state between the DABCO 3s electron and the solvent. Red-shifted transition origins are observed for DABCO/amine, ether, aromatic, and acetonitrile clusters. Although significant, the anticipated dipole-induced dipole interaction is not sufficient to explain these substantial red shifts. The large stabilization of the excited Rydberg state in these latter clusters is proposed to arise from a delocalization of the DABCO 3s Rydberg electron into the comparable energy 3s or other orbitals of the solvent; that is, the cluster stabilization is at least partially due to an electron-transfer interaction. This mechanism for enhanced cluster excited-state interaction is especially important for solute/solvent systems with 3s Rydberg orbitals at comparable energies.