The diffusion of the 235NpO2+ and 95m TCo4− -ions has been measured directly in sample cylinders of two different sediments from the floor of the deep sea. In smectiterich sediment not shielded from contact with atmospheric oxygen the following values were obtained for the effective diffusion coefficients of neptunyl- and pertechnetateions, respectively: Deff(NpO2+) = 1.5 × 10−12 m2 s−1 and Deff(TcO4−) = 3.2 × 10−10 m2 s−1. Under anoxic conditions in sediment with known reducing properties, the pertechnetate ion appears to undergo slow reduction and the effective diffusion coefficient of the reduced species of Deff(Tc,red)= 1.1 × 1014 m2 s−1 reflects a substantial decrease of the mobility of the lower-valent technetium.