Modern statistical analysis methods were employed to investigate and evaluate the effect of chemical composition on mechanical properties of interpenetrating polymer network (IPN) formulations that had been synthesized of polyurethane and unsaturated polyester resin (cross-linked with styrene). Experiments run according to the mathematical plan made a basis for empirical models derived from the Sheffe polynomials. The models were utilized to optimize the chemical composition of IPN from the viewpoint of achievable mechanical parameters, to assess changes of those properties over the whole theoretically possible scope of quantitative compositions, and to investigate the effects of individual components and their possible interactions. The findings supported a thesis on complex interactions between polymeric components of IPN; they can be the reason for tangled polyurethane, unsaturated polyester, and linear polystyrene polymer networks, and ultimately they can be responsible for the formation of immiscible phases within the plastics produced. Data were also obtained which make it possible to evaluate potential mechanical properties of synthesized IPN compounds as coating materials and engineering plastics. (C) 2005 Wiley Periodicals, Inc.