Conventional tensile dilatometry techniques are extended to cyclic fatigue applications to study volume changes that occur during controlled-load cyclic fatigue of polyacetal. During fatigue, in-situ measures of the irreversible and elastic volume change are monitored together with dynamic viscoelastic parameters (E’, E ", and Tan delta], and changes in the energy densities (strain energy, potential energy, and irreversible work). The results show that the effective irreversible volume of the polyacetal gradually increases over a wide range of applied cyclic stress. However, at high stress levels and/of frequencies (i.e., low-cycle, thermally dominated regime), the effective Poisson’s ratio of the polyacetal increases as it softens (evidenced by the dynamic viscoelastic data). Conversely, at lower stress levels, the Poisson’s ratio continually decreases coincident with decreases in the loss modulus (E ") and the irreversible work density. These results are indicative of entirely different mechanisms governing the low-cycle (high stress level] and high-cycle (low stress) regimes, Also, comparisons between tensile and fatigue dilatometry studies show that the dilational-strain response of samples fatigued at high stress levels are similar to data obtained from monotonic tensile dilatometry. However, the dilation-strain response of samples fatigued at lower stress levels are distinctly different from low-cycle fatigue and tensile dilatometry.