Thermodynamic properties and equilibrium constant of reaction in nanosystems were analyzed theoretically. The effects of sizes of nano-CuO on thermodynamic properties and equilibrium constant were studied using the reaction of nano-copper oxide and sodium bisulfate as a system. The experimental results indicate that with the sizes of reactant decreasing, the molar Gibbs free energy (Delta(r)G(m)), the molar enthalpy (Delta(r)H(m)) and the molar entropy (Delta(r)S(m)) decrease, but the equilibrium constant (K) increases and there are linear trends between the reciprocal of sizes for nano-CuO and the values of Delta(r)G(m), Delta(r)H(m), Delta(r)S(m) and Ln K, which are in agreement with the theoretical analysis. (C) 2011 Elsevier Ltd. All rights reserved.