Fatigue tests in reversed tension-compression were carried out on the sample of polystyrene (PS), poly(oxy-2,6-dimethyl-1,4-phenylene [poly(phenylene ether) (PPE)], and their alloys. The dynamic loss modulus, storage modulus, tan delta, and surface temperature of the specimens were continuously measured by a viscoelastometer during the fatigue tests. Fatigue lives and analysis of the samples by electron microscopy were also measured. The mechanical work, heat generated, and percent of work that appears as heat (PWAH) were calculated by the equations derived by viscoelastic equations and heat transfer. PWAH was in the range from 40 to 65%. The fatigue lives and PWAH were longer and larger for PPE and PPE-rich alloys than for PS and PS-rich alloys, respectively. The cleavage energy of the polymer chain calculated from the decrease of molecular weight before and after the fatigue test were much smaller compared to the total mechanical work. The entanglement densities observed before and after the test were the same. The majority of the work due to structural deformation is thought to be consumed by formation of striation and microscopically irreversible deformation.