Catalysis by Cu salts (MnCuCln+1; M = K+, NH4(+)) in concentrated aqueous solutions (approximately 5-8 M) is investigated using a flow reaction system at 75 degreesC. Selective dimerization of acetylene gives vinylacetylene as a major product and much smaller amounts of 1,2-divinylacetylene, acetaldehyde, 1-ethynyl-1,3-butadiene, and uncharacterized black tarry products. Cooling the catalyst solution to room temperature causes the growth of a mixture of tetrahedral and needle-like colorless crystals. Observation of the crystals by microscopy and their crystallography, and analyses of the Cu compounds reveal that they are CuCl and K2CUCl3, respectively. Plots of the graphs of the solubility of the Cu salts in water as a function of temperature and composition of the salt show the dominant presence of KCUCl2 in the solution. A mechanism of the reaction is proposed based on the performance of the catalyst and its structure.