The aim of the research program "Theoretical Chemistry" was to contribute toward better understanding of chemical systems on the molecular level. To this goal, theoretical and experimental methods were applied. The main topics of the proposal were the following. a) The study of highly asymmetric electrolytes as models for solutions of globular proteins, surfactant micelles, and colloidal suspensions. It was experimentally observed that correlation among multivalent counterions cause a nonuniform distribution of macroions in solutions. We investigated the origin of this phenomenon and we proposed an explanation on molecular level. b) The properties of solutions adsorbed in random nanoporous materials (e.g., aerogels) differ substantially from the bulk properties of solutions. Our goal was to calculate the thermodynamic properties of adsorbed electrolytes in equilibrium with a corresponding bulk solution. We developed the necessary theoretical equations to describe these partly - quenched systems. c) In protein solutions strong short- ranged forces are present which may lead to association of protein molecules. A theory to describe such systems was proposed; the theoretical calculations were compared with our experimental results obtained with the small angle X- ray scattering (SAXS) and with osmotic measurements. d) In order to understand the biological processes it is crucial to understand the role of water in these systems. We studied the hydration of hydrophobe molecules and by using a simple model we explained the influence of ions on the solubility of hydrophobe molecules (Hofmeister series). In addition to computer simulations the newly developed integral equation theory was applied to the same model. e) Integral equation theories and computer simulation were used to study the distribution of binary mixture between the planar slit and the homogeneous phase. This quantity is strongly dependent on the difference in the strength of interparticle attractive interactions, which can even lead to the decomposition of mixture. f) Small-angle X-ray scattering technique was used to examine the structural properties of (i) microemulsions that could be used as reaction media for enzyme catalyzed reactions, and (ii) pharmaceutically usable microemulsions that are applied as drug delivery systems. Experimental data were analyzed using the generalized indirect Fourier transformation method, which made possible the data treatment of concentrated systems with interacting scattering particles. g) Polyelectrolyte solutions exhibit nonideal behavior even at very low concentrations. A source of these characteristic properties is the strong electrostatic interaction between macroions and the counterions. Using SAXS we studied the polyion-counterion correlation in the solutions containing the mixture of different counterions. It was confirmed that the correlation y depends strongly on the concentration and valence of counterions, as also on the concentration of added low molecular salt. The majority of the results (from 1999 to 2003) were published in 39 papers in scientific journals with high impact factor (Annu. Rev. Phys. Chem, J. Am. Chem. Soc., Biophys. J., J. Phys. Chem. B, J. Chem. Phys., and others). The members of the group had given invited talks at international conferences and foreign universities. The program "Theoretical Chemistry" was during this period financed with 1 FTE.