BACKGROUND: Loss of the active CuCl component occurs during acetylene dimerisation to monovinylacetylene (MVA) catalysed by an anhydrous catalyst with the formation of a dark red precipitate. Acidic species can reduce the loss of CuCl but have an unfavorable influence on acetylene dimerisation. This study aims to determine the precipitate composition and regulate the acidity of the catalyst to find a balance between reaction rate, MVA selectivity and catalyst life. RESULT: The precipitate composition was 2CuCl center dot 3C2H2 center dot 1/3CH3CH2NH2 center dot 1/7C3H7NO, formed by the combination of DMF, CH3CH2NH2, C2H2 and the [Cu]-acetylene -complex, which is an intermediate in the reaction. From an overall consideration of the loss of CuCl, conversion of acetylene, and selectivity of MVA, the reaction temperature and acetylene space velocity were optimized at 65 degrees C and 200 h1, respectively. The introduction of HCl into the catalyst with a rate of 3.2 h1 could reduce CuCl loss by 73.5%, whereas conversion of acetylene was only lowered by 9.0%. CONCLUSION: Acidity regulation of the anhydrous catalyst by optimising the reaction temperature, acetylene space velocity, and rate of addition of HCl shows little negative effect on acetylene conversion and selectivity to MVA but can reduce CuCl loss significantly. (c) 2012 Society of Chemical Industry