In the reported experiments, the stress softening behavior of a carbon black filled butyl rubber in cyclic loading (creep) and cyclic deformation (stress relaxation) histories was investigated. The data were analyzed in view of the BKZ constitutive equation. For materials which follow the behavior predicted by the BKZ theory, the single step stress relaxation response is a lower bound for the stress in cyclic deformation histories, and the single step creep response is an upper bound for the deformations obtained in cyclic loading histories. It is suggested that an adequate description of material behavior might be obtained using a theory similar to the BKZ theory, which includes competing effects of stiffening in creep and softening under load-unload cycling. The failure lifetime of the filled butyl rubber in cyclic loading histories was related to the stress softening behavior. The researchers were able to predict failure lifetimes by using creep data obtained from short time cyclic loading histories and extrapolating to the strain at break determined from a shifted failure envelope. This approach relates the observed frequency and waveform dependence of failure lifetimes to the creep behavior under cyclic loading conditions.