The behavior of 226Ra in the Mississippi River mixing zone is strongy nonconservative and includes desorption similar to that for the Hudson, Pee Dee, and Amazon rivers. However, dissolved and desorbed 226Ra concentrations in the Mississippi are 2 to 5 times greater than in the other rivers at the same salinity. Radium concentrations vary inversely with the water discharge rate. The 226Ra desorption maximum occurs at a salinity of 5.0, much lower than the 18 to 28 salinity values for the maxima of the other three rivers. High concentrations of dissolved 226Ra (up to 82 dpm per 100 L) and the low salinity values for the desorption maximum in the Mississippi River result from three major factors. Suspended sediments include a large fraction of montmorillonite, which gives the sediment a high cation exchange capacity. 0.54 meq/g. The average suspended sediment load is large, about 510 mg/L, and contains 1.9 dpm g desorbable 226Ra. The dissolved 226Ra river water end-member (9.6 dpm per 100 L) is higher than in surface seawater. The annual contribution of 226Ra to the ocean from the Mississippi River is 3.7¿1014 dmp/yr based on data from three cruises. Evidence of flux of 226Ra from estuarine and shelf sediments is common in vertical profile sampling of the deltaic waters but is not reflected in calculations made with an ''apparent'' river water Ra value extrapolated to zero salinity.