Analyses of rubidium, strontium, and strontium isotope composition were made on a suite of whole, surface-sediment, carbonate-free samples from the Plata estuary, its major source rivers, and the adjacent continental shelf. Rubidium and strontium analyses were also made on the greater-than-20-micron and less-than-20-micron fractions of the samples to determine the effect of size frequency distribution on one of the parameters. Because the Rb to Sr ratio, and presumably the Sr87 to Sr86 ratio, varies with grain size, some of the large variability in Sr87 to Sr86 in the Plata must be due to differential sedimentation processes within the estuary. Consideration of the data on an isochron diagram reduces some of this ambiguity and is consistent with an interpretation of two major sources of sediment in the estuary. These sources are the rivers Parana and Uruguay at the head of the estuary and Argentine continental shelf sediment at the mouth. A third source, Uruguayan shield material reported by Urien (this volume) along the estuary’s north shore, is not distinguishable in the data presented here.