Using a modified torsional dynamic rheometer, strain and torque signals are captured and analyzed through Fourier transform. A thorough analysis of strain signal is first performed to document the quality of the applied deformation and to define the optimum conditions for signal capture, and hence, for Fourier transform calculation. A specific calculation procedure is developed that allows the error associated with FT calculation to be estimated, in terms of mean harmonic peaks and their standard deviation. Scatter is observed on low strain results that nearly vanishes when the applied deformation is larger than 14%, thus giving confidence in data gathered under nonlinear viscoelastic conditions. Results on a series of gum EPDM with different macromolecular characteristics (MWD and long chain branching, LCB) are reported and discussed, with respect to a simple four parameters model for the variation on deformation of the relative third harmonic component. Processing additives are found to mask expected LCB effects. Results on a series of SBR 1500 samples, collected from various manufacturers, are presented that illustrate the capability of FT rheometry to detect differences between materials expected to be similar. (C) 2003 Wiley Periodicals, Inc.