Aluminum phosphate sulfate (APS) minerals associated with the Athabasca Basin are compositionally part of the alunite group. Typically 5–20 μm in size, they accompany the regional diagenetic-hydrothermal illite-kaolinite-dickite assemblage in the Athabasca Group sandstones, the illite-sudoite assemblage of the altered paleo-regolith, and alteration zones surrounding uranium deposits. In the vicinity of the Centennial deposit, where illite and sudoite replace coarse-grained aluminosilicate porphyroblasts in the basement phyllitic pelites, cubic APS crystals are as large as 80 μm. Detailed petrography indicates that the APS crystals form broadly coeval to illite. Backscattered electron imaging, elemental mapping, and compositional analysis reveal complex zoning as well as later hydrothermal alteration of the APS crystals. Growth-zoned crystals are a solid solution between crandallite-goyazite-svanbergite [Sr0.17–0.32Ca0.0.18–0.28LREE0.0.25–0.46(Al2.78–2.92Fe0.01–014)(PO4)1.82–2.03(SO4)0.14–0.35(OH)6], whereas the later alteration of the APS minerals results in a compositional shift to endmember florencite [Sr0.15Ca0.11LREE0.57(Al2.86Fe0.02)(PO4)1.98(SO4)0.15(OH)6]. Fluids responsible for the alteration of APS to florencite are paragenetically linked to late hydrothermal fluids associated with mafic Mackenzie dikes and do not appear to be related to proximity to uranium mineralization. Both zoned and altered portions of the crystals have bulk compositions that overlap with APS minerals in other areas of the basin suggesting a common genetic origin. However, it is critical to link the paragenetic context to observed compositional changes. An increase in MREE from early to late stages of zoned crystal growth correlates with the greatest concentration of REE found in uraninite from unconformity-related uranium deposits. This could be a link between broader APS growth and uraninite precipitation.