The nonlinear viscoelastic properties of two series of highly loaded carbon black natural rubber composites with filler volume fraction in the 0.148-0.309 range were investigated at 100 degrees C through strain sweep tests at 0.5 and 1.0 Hz frequency, up to strain amplitude of around 1000%, using a commercial torsional dynamic rheometer adapted to perform so-called Fourier Transform rheometry experiments. A series of high cis-1,4 polybutadiene (BR)/ carbon compounds with filler fractions in the 0-0.213 range was also tested for comparison. Except BR compounds with carbon black fractions below the so-called percolation level (0.12-0.13), nonlinear viscoelastic responses were systematically observed within the experimental strain window (6-1000%). Fourier Transform treatment of recorded torque and strain signals allowed to express the material response in terms of harmonics, with the main one (i.e., at to the test frequency) corresponding to the complex modulus. The usual drop of complex modulus with increasing strain was observed and adequately analyzed with a model previously reported. The expected effect of carbon black content was observed. Highly loaded samples were found to exhibit a variation of relative torque harmonics with strain amplitude markedly different from unfilled rubbers, which lead to the development of a model, inspired by the Weibull analysis. This model explicitly considers that filler particles, dispersed in the rubber matrix, self-organize in a structure that adds a nonlinear response to the growing nonlinearity exhibited by the polymer matrix as applied strain increases. Above a critical strain, easily determined by mathematically handling model parameters, the filler structure dislocates and the high strain nonlinear response of the matrix is asymptotically recovered. (c) 2008 Wiley Periodicals, Inc.