In this paper we present the results of uniaxial, equal biaxial, and unequal homogeneous biaxial tension of viscoelastic materials under isothermal conditions and compare them with predictions based on the approximate constitutive equations of Refs. 1 and 2. Theoretical expressions for uniaxial and equal biaxial constant and logarithmic stretch‐rates and for unequal homogeneous biaxial single‐step and double‐step relaxation and constant stretch‐rate are developed and compared with experimental results for a styrene‐butadiene rubber (SBR); agreement is found to be satisfactory. The uniaxial and equal biaxial constant stretch results for very short ramp times are used to predict the behavior of actual single‐step relaxation tests and to study the effects of fast motion on the constitutive equations of Refs. 1 and 2.