Fourier transform rheometry was used to investigate the nonlinear viscoelastic behavior of a series of gum polybutadiene and styrene-buta diene rubbers. A commercial torsional rheometer [the rubber process analyzer (RPA(R), Alpha Technologies)] was suitably modified for capturing strain an torque signals, using appropriate software developed in Lab View(R) (National Instruments). Proprietary programs, written in MathCad(R) (MathSoft Inc.), were used to perform Fourier transform (FT) calculations and other data treatments. Results showed that FT rheometry provides not only the same information as standard dynamic testing in the linear region, although with a better accuracy, but gives valuable information about the nonlinear behavior of materials. Using a four-parameter equation to model the variation of the relative 3rd-harmonic component with strain amplitude, it was found that the strain sensitivity of the materials was adequately quantified, with results apparently consistent with their processing shear sensitivity. The only material parameters that were found to significantly affect the strain sensitivity are the chemical nature of the elastomers (i.e., polybutadiene versus styrene- butadiene rubber) and the molecular weight. No clear effect of the microstructure was seen, however, likely because the sampling was not discriminating enough in this respect. (C) 2004 Wiley Periodicals, Inc.