Glendonites, calcite pseudomorphs after ikaite (CaCO3 • 6H2O), feature prominently in fine-grained, glacially influenced Permian marine strata of eastern Australia, which accumulated in a series of extensional basins that occupied polar to temperate latitudes along the southeastern margin of Gondwana. Because ikaite formation in the marine environment requires near-freezing temperatures, high alkalinity, and elevated concentrations of orthophosphate, the presence of glendonites in ancient strata implies a particular array of paleoenvironmental conditions. A petrographic and geochemical study was carried out to assess the veracity of isotopic data from glendonites as proxy indicators of ancient ocean chemistry, sea-floor temperature, and diagenesis. In thin section, glendonites consist of inclusion-zoned, equant to bladed calcite crystals, interpreted as the ikaite replacement phase, enclosed in a matrix of calcite and minor chalcedony and pyrite cements. Carbon and oxygen isotope data show a negative covariance, with δ18O values ranging from −22.5 to +0.3‰ and δ13C values from −27.7 to −4.5‰ V-PDB. Of the calcite phases examined, the ikaite replacement phase is the most enriched in 18O and most depleted in 13C; enclosing cements possess lower δ18O and higher δ13C values and are interpreted to have formed during later stages of burial diagenesis. The δ18O values of the ikaite replacement phase do not correspond to formation with Permian seawater at near-freezing temperatures unless isotopic disequilibrium is assumed. The δ13C values are similar to the carbon isotope composition of bulk organic matter in the host sediment, and suggest that the alkalinity and orthophosphate necessary for ikaite stability were generated in the zone of suboxic to anoxic diagenesis. Results indicate that although δ18O and δ13C values from glendonites are useful for understanding early to late diagenetic processes, they are not ideal proxies for seawater chemistry and temperature.