The material nonlinearities in the viscoelastic behavior of rubbers at modest-to-large deformations have been studied. A vulcanized network based on the styrene-butadiene elastomer has been tested in stress-relaxation experiments at room temperature for a wide range of deformations. After time-strain separabality was confirmed, the damping function could be calculated and introduced into the Boltzmann superposition integral. The equilibrium stress-strain behavior of the system could then be described, while comparison with other theoretical equations has also been made.