Late Pennsylvanian (upper Gzhelian = Virgilian) to early Permian (lower Artinskian = upper Wolfcampian) sea water chemistry and depositional cyclicity are evaluated based on oxygen and carbon isotopic compositions of unaltered shells of the brachiopodComposita collected from shallow-water deposits in the Council Grove and Chase Groups in Kansas and Oklahoma. Mean MgCO3 content in these samples is 0.2±0.2 mole%, and mean δ18O pdb and δ13C pdb values are −2.1±0.6‰ and 4.0±0.8‰, respectively. There is no significant difference in mean percentage MgCO3 or isotopic compositions between primary prismatic and secondary fibrous shell layers. δ18O trends do not appear to coicide with inferred sea water temperature changes attending deposition of stratigraphic sequences as has been postulated for older midcontinent Pennsylvanian cyclic deposits. Rather, the data seemingly reflect variations in sea water δ18O values coincident with ice-volume fluctuations during the time period examined. This contention is in accordance with inferred glacio-eustatic forcing of depositional cyclicity in the section. Estimated amplitudes of sea-level fluctuations increase from the upper Gzhelian into the lower Artinskian from 30–100 m to 110–150 m, which concurs with maximum Gondwanan glaciation during the early Permian. Although there is not a significant long-term trend in mean δ13C values, there is an upward trend to less negative δ18O values that is interpreted as a signal of long-term increase in midcontinent aridity and salinity.