The diffusion of indigenous nitrate from small (< 5 mm) aggregates of a clay-loam soil, and added bromide from larger (30-70 mm) clods of a sandy clay, were studied under saturated conditions. The time (20.5) taken for half the diffusional solute loss to occur from the small aggregates was well related to the square of the aggregate radius (a2). The impedance factor Ui, of 0.54 calculated from the t0.5 v. a2 relationship gave satisfactory results in simulations of the measured Mt/M, v. time curves.L0.5 was also well related to a2 for the clods, but thefc values calculated from it were not only very large (>0.9) but also decreased with increasing clod size, possibly because of anion exclusion effects.The model simulated bromide diffusion from chalk cubes of mixed sizes better when computations were made for each size than when a single volume-averaged size was taken.