U-Pb ages of paleosol samples from cyclothemic sections in the Sacramento Mountains of New Mexico and the Permian basin of West Texas bracket the Pennsylvanian-Permian and Carboniferous-Permian boundaries. Linear interpolation and extrapolation to boundaries are accomplished by using cyclothems as periodic subdivisions. The Carboniferous-Permian boundary is 301 ± 2 Ma (2 σ), the Pennsylvanian-Permian boundary is 302 ± 2.4 (2 σ), and the Missourian-Virgilian boundary is 307 ± 3 Ma (2 σ). These ages are within the uncertainties of recent estimates for the boundaries but are significantly more precise. The average cycle duration for this Upper Pennsylvanian and Lower Permian interval is 143 ± 64 ka (2 σ), within the uncertainty of the ∼100 k.y. cycle that dominated Pleistocene sea-level history. This new estimate, which is much shorter than those published previously, suggests that the Pennsylvanian and Early Permian cyclothems may have been driven in a manner similar to that which caused glacio-eustatic cycles of the Pleistocene.