Tracer tests were performed in distinct biogeochemical zones of a sand and gravel aquifer in Cape Cod, MA, to study the redox chemistry (I) and transport (Cs, I) of cesium and iodine in a field setting. Injection of iodide (I(-)) into an oxic zone of the aquifer resulted in oxidation of I(-) to molecular iodine (I(2)) and iodate (10(3)(-)) over transport distances of several meters. Oxidation is attributed to Mn-oxides present in the sediment Transport of injected 10(3)(-) and Cs(+) was retarded in the mildly acidic oxic zone, with retardation factors of 1.6-1.8 for 10(3)(-) and 2.3-4.4 for Cs. Cs retardation was likely due to cation exchange reactions. Injection of 10(3)(-) into a Fe-reducing zone of the aquifer resulted in rapid and complete reduction to I(-) within 3 m of transport. Then on conservative behavior of Cs and I observed during the tracer tests underscores the necessity of taking the redox chemistry of I as well as sorption properties of I species and Cs into account when predicting transport of radionuclides (e.g., (129)I and (137)Cs) in the environment.