The synthesis and properties of carbon-black-reinforced natural rubber/polystyrene full interpenetrated polymer networks were studied in detail. The interpenetrated polymer networks were prepared with the sequential method. The natural rubber was reinforced with 30 phr carbon black and crosslinked with 4 phr dicumyl peroxide. In all cases, natural rubber was the dominant phase in the interpenetrated polymer networks. When the polystyrene content of the interpenetrated polymer networks was increased to 12%, a good improvement in the tensile properties was observed, and the strength dropped with greater polystyrene content. Dynamic mechanical thermal analysis showed that when the tensile strength improved, there was a single phase, and a broad glass-transition temperature peak was observed in a plot of the loss factor versus the temperature. On the contrary, when the tensile strength dropped, there was phase separation, and two glass-transition temperature peaks were observed in a plot of the loss factor versus the temperature. In fact, there was a good relation between the static properties and dynamic properties, so we could predict static properties such as the tensile strength by an analysis of the dynamic properties. It was revealed that the phase separation of natural rubber and polystyrene depended on the component ratio and carbon black content. Also, the presence of carbon black reduced the miscibility of natural rubber and polystyrene. Thermogravimetric analysis showed that the interpenetrated polymer networks, regardless of their composition, had better thermal properties than the corresponding homopolymers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2143-2149, 2009