A mathematical model is presented which describes the distribution of radon 222 in sediments having a constant or variable depth distribution of radium 226. The model is extended to the distribution of lead 210, taking into account the mobility of radon (the precursor of 210Pb) within the sediment column. The 210Pb model is compared, at constant radium activity, with the conventional approach which disregards the radon diffusion when estimating sedimentation rates by the 210Pb method. The ratio between apparent and real sedimentation rate, s'/s, expressed as a function of three dimensionless parameters, demonstrates the importance of the radon diffusion effect. This effect is particularly important for sediments with small initial excess 210Pb activity, small sedimentation rate, large radon diffusivity, or a combination of these factors. Applied to Lake Geneva, the sedimentation is estimated to be larger by 30--50% than the original value by Krishnaswami et al, (1971). In sediments which are mixed at the surface (physical mixing or bioturbation), the 210PB activity in the mixed layer is diminished compared to that in the settling sediment material (Robbins et al., 1977), and radon diffusion makes the activity difference even larger, especially for low initial excess 210Pb activity, small sedimentation rate, and large mixing intensity. This result may be of importance for the balance of 210Pb in an aquatic system if the calculations are based on activities measured in the sediment.