Coppet chloride catalysis is a well-established field in organic and inorganic chemistry. However, in most cases a detailed mechanistic understanding of the individual reaction steps and identification of reactive intermediates are still missing. The present study reports the results of spectroscopic and spectrometric measurements that support formation of copper agglomerates during catalytic processes. The composition of CuCl2 center dot 2H(2)O in several coordinating solvents and the influence of, asic coreagents such as (NaOBu)-Bu-t and K2CO3 on the structure in the solid state as well as in solution were investigated. Several experiments involving crystal structure determination, IR spectroscopy, and ultra-high-resolution cryospray-ionization mass spectrometry were performed. The crystal structures of [CuCl2(H2O)]center dot 0.5(CH3)(2)CO (1), [Cu-2(CH3CN)(2)Cl-4] (2), [Cu-3(CH3CN)(3)Cl-6] (3), [Cu3Cl6(THF)(4)] (4), [Cu(DMSO)(2)Cl-2] (5), (H2N(CH3)(2))(2)[CuCl3] (6), and [Cu4OCl6(THF)(urea)(3)]center dot 3THF center dot urea (8) are reported herein. It can be clearly demonstrated that it mu(4)-oxido copper clusters of the formula [Cu4OCl6(solvent)(4)] are the main product from the reactions of CuCl2 center dot 2H(2)O and basic coreagents. As a final result of these experiments, it can be stated that mu(4)-oxido copper clusters most likely play an important role in the mechanism of copper chloride-catalyzed reactions.