Relative rate constants for the mutual neutralization of hydrated hydronium and hydrated hydroxide cluster ions have been measured using a new ion optical method which merges mass-selected beams of oppositely charged cluster ions with an electrostatic quadrupole deflector. Measurements are presented for all combinations of the reaction, H3O+(H2O)(n=0-3) + OH-(H2O)(m)=(0-3), under one ion optical setting of the interaction region (different relative velocities) and a few representative reactions as a function of relative velocity. A pronounced enhancement in the relative rate constant is seen with clustering which suggests that cluster specific rate constants may be required for accurate chemical modeling of the D region of the ionosphere. The neutralization reaction mechanism changes dramatically with clustering which is interpreted as st change from electron transfer at small cluster size to proton transfer with more clustering.