A small-angle X-ray scattering (SAXS) and swelling study of natural rubber and styrene-butadiene rubber blends (NR/SBR) is presented. To this aim, specimens of NR and SBR and blends with 75/25, 50/50, and 25/75 NR/SBR ratios (in phr) were prepared at a cure temperature of 433 K and the optimum cure time (t(100)). This time was obtained from rheometer torque curves. The system of cure used in the samples was sulfur/n-t-butyl-2-benzothiazole sulfenamide. From swelling tests of the cured samples, information about the molecular weight of the network chain between chemical crosslinks was obtained. For all cured compounds, in the Lorentz plots built from SAXS scattering curves, a maximum of the scattering vector q around 0.14 angstrom(-1) was observed. However, the q position shows a linear-like shift toward lower values when the SBR content in the SBR/NR blend increases. In pure NR or SBR the q values show a different tendency. The results obtained are discussed in terms of the existence of different levels of vulcanization for each single phase forming the blend and the existence of a third level of vulcanization located in the interfacial NR/SBR layer. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2320-2327, 2009472323202327LNLS, Brazil [D11A-SAXS1 4181/05]Agencia Nacional de Promocion Cientifica y Tecnologica [PID 2003-0435, PICT 2006-1650]Comision de Investigaciones Cientificas de la Provincia de Buenos AiresSECAT (UNCentro), ArgentinaUniversidad de Buenos Aires, Argentina [X808]LNLS, Brazil [D11A-SAXS1 4181/05]Agencia Nacional de Promocion Cientifica y Tecnologica [PID 2003-0435, PICT 2006-1650]Universidad de Buenos Aires, Argentina [X808]