The dealloying behavior of single-phase Mn-30Cu sheets in acetic acid solutions with varying concentrations ranging from 1 vol. % to 60 vol. % and morphological evolution of nanoporosity have been investigated. The results show that the concentration of acetic acid solution has a key influence on the dealloying process and microstructure of nanoporous copper. It can be well explained as a consequence of the competition between association and ionization of acetic acid molecules. Additionally, the formation rate of porous structure in organic acid solution is remarkably lower than that in inorganic acid due to the limited H+ concentration from the competitive effect.