Styrene is one of the comonomers most frequently used in the curing of unsaturated polyester resins. The use of nonazeotropic compositions leads to the formation of networks, in which the styrene-polyester ratio varies significantly during curing, as shown through FTIR spectroscopy. Dynamic mechanical thermal analysis (DMTA) showed how the variation in the styrene content affects the network structure that is formed. The results showed a decrease in network density in the systems in which the azeotropic composition was not used. The styrene content is therefore a factor that governs the curing process, in addition to the ultimate properties of the cured resins. (C) 2003 Wiley Periodicals, Inc.