A new apparatus for inducing changes in the properties of water in closed dielectric vessel by subjecting it to pulsed direct current glow discharge plasma is designed and constructed. It has been hypothesized that the action of plasma on the structure of water consists in resonance excitation of water aggregates. As a result of resonance excitation, aggregates of high molar masses are broken down into low molecular mass aggregates. Analysis of the ESI MS spectra revealed that in all tested aqueous solutions after exposure to plasma, the concentration of low-molecular solvated ions [M(H2O)]+ and [M(H2O)2]+ significantly increased, while the concentration of the ions of high molecular masses [M(H2O)6-10]+ solvated by water aggregates decreased, relative to their concentrations in the water solutions not subjected to plasma irradiation. According to our measurements also a significant change in pH occurs. The presented results clearly show that it is possible to process a liquid that changes its structure without involving high processing energy and, unexpectedly, the obtained change of parameters is significant and stable over time.