Effective diffusion coefficients for selected radionuclides have been measured in ocean floor sediments to provide data for the assessment of barrier effectiveness in subseabed repositories for nuclear waste. The sediments tested include illite-rich and smectite-rich red clays from the mid plate gyre region of the Pacific Ocean, reducing sediment from the continental shelf of the northwest coast of North America, and Atlantic Ocean sediments from the Southern Nares Abyssal Plain and the Great Meteor East region. Results show extremely small effective diffusion coefficients with values less than 10−14 m2s−1 for plutonium, americium, curium, thorium, and tin. Radionuclides with high diffusion coefficients of approximately 10−10 m2s− include the anionic species pertechnetate, TcO4
−, iodide, I−, and selenite, SO3
−2. Uranyl(VI) and neptunyl(V) ions which are stable in solution, have diffusion coefficients around 10−12m2s−1. The diffusion behavior of most radionuclides is similar in the oxygenated Pacific sediments and in the anoxic sediments from the Atlantic. An exception is neptunium, which is immobilized by Great Meteor East sedi-ment, but has high mobility in Southern Nares Abyssal Plain sediment. Under stagnant conditions a 30 m thick sediment layer forms an effective geologic barrier isolating radionuclides in a subseabed repository from the biosphere.