A mechanistic, two-layer mass balance model for dissolved oxygen (DO) in Onondaga Lake, a hypereutrophic, urban system, has been developed and tested. The model accommodates the processes of reaeration, algal photosynthesis and respiration, carbonaceous biochemical oxygen demand (CBOD), nitrogenous biochemical oxygen demand (NBOD), sediment oxygen demand (SOD) and water column oxidation of reduced species. Most model inputs, and many of the kinetic coefficients, were developed through a program of field monitoring and field and laboratory experimentation. The model was calibrated and verified using data collected in 1989 and 1990, respectively. Severe dissolved oxygen depletion is observed in the hypolimnion (lower mixed layer; LML) of Onondaga Lake during summer stratification. SOD and water column oxidation of reduced species diffusing from the sediment are together responsible for >70% of the summer depletion; CBOD, NBOD and algal respiration account for the balance. DO depletion occurs in the epilimnion (upper mixed layer; UML) in the fall. Oxidation of reduced species entrained from the LML is responsible for about 30% of the fall depletion, with algal respiration and mass transport to the LML accounting for about 25% each; CBOD and NBOD are minor sinks. Beyond those features of pollutant loading, fate, and transport, sensitivity analyses suggest mat meteorological conditions importantly influence the character of the DO resources of the lake.