Lake Erie’s central basin experiences seasonal anoxia, contributing to internal sediment phosphorus (P) loading and exacerbating eutrophication. The precise conditions required for internal loading are poorly understood. This study constrains the timing and rates of internal P loading using continuous in situ temperature, dissolved oxygen (DO), and soluble reactive P (SRP) observations from two sites. SRP concentrations remained low during normoxia (>2 mg of DO L–1) and hypoxia (0–2 mg of DO L–1) but increased abruptly after anoxia for 12–42 h. SRP flux rate estimations varied, likely due to advection and hypolimnion thickness variation, but could still be reasonably quantified. Flux rates and standard errors during anoxia averaged 25.67 ± 5.5 mg m–2 day–1 at the shallower site and 11.42 ± 2.6 mg m–2 day–1 at the deeper site. At the shallower site, the anoxic hypolimnion was displaced with normoxic water, causing cessation of P flux until anoxia returned, and higher flux rates resumed immediately (89.1 ± 8.6 mg m–2 day–1), suggesting rapid, redox-controlled P desorption from surface sediments. On the basis of our rate and onset findings, the expected anoxic area and duration in the basin could yield an annual internal SRP load comparable to the annual central basin TP tributary load of 10000–11000 metric tonnes.