Oxalate (C2O42-) is a common dianion, but it is not electronically stable as an isolated species due to the strong intramolecular Coulomb repulsion and can only exist as solvated species. We observed hydrated oxalate clusters, C2O42-(H2O)(n) for n=3-40, using electrospray ionization of an oxalate salt solution and studied their energetics and stabilities using photodetachment photoelectron spectroscopy and theoretical calculations. We found that the smallest observable solvated cluster, C2O42-(H2O)(3), has an adiabatic electron binding energy of similar to0.0 eV, i.e., a minimum of three H2O is required to stabilize C2O42- in the gas phase. Theoretical calculations show that the first four waters bind tightly to C2O42-, each forming two H-bonds with C2O42- peripherally without interwater H-bonding. The charges of the dianion were stabilized sufficiently that additional waters beyond n=4 form only single H-bonds with C2O42- and interwater H-bonding was observed starting at n=5. The repulsive Coulomb barrier, characteristic of multiply-charged anions, was estimated from photon energy-dependent spectra for the smaller clusters and was found to decrease with increasing n. We observed that photoelectron intensities for features of the solute decreased as n increased, whereas detachment signals from the solvent became dominant for the large solvated clusters. This observation suggested that C2O42- is situated in the center of the solvated clusters so that electrons detached from the solute were suppressed by the surrounding solvent layer. (C) 2003 American Institute of Physics.