Nearly all of the acetylene reactions under pressure are performed in the liquid [20]. In this reaction, acetone is used as solvent and H2O is one of the reactants. The effect of volume ratio of ace-tone to H2O on the hydrocarboxylation of acetylene to acrylic acid is shown in Fig. 2. The yield, selectivity and STY enhanced drastically with increase in the acetone/H2O volume ratio. The highest yield of 72.1% and STY of 764.6 mol AA/(mol Ni h) were obtained at an acetone/H2O volume ratio of 10, the corresponding selectivity to acrylic acid is 85.2%. However, the activity decreased with further increase in the acetone/H2O volume ratio. For example, when 6.5 ml of water was mixed with 158.5 ml of acetone, resulting in 24.4 of acetone/H2O volume ratio, the STY decreased to about 314.3 mol AA/(mol Ni h). Two main reasons for this phenomenon are proposed as follows. On one hand, the solubility of CO and C2H2 in the water is very low and there are not enough gas reactant molecules that can work with catalysts if abundant water was added in the reaction system, thus causing the decrease in the reaction rate. Due to its poor solubility, the partial and total pressure of gas would increase and result in the increase of side reaction. On the other hand, water acts as a reactant, a certain amount of water can enhance the formation rate of Ni H active species and meanwhile promote its attack to Ni-acyl complex and thus favors the formation of acrylic acid. The shortage of water is a disadvantage to the effective performance of catalytic cycle.