In retrospective radon measurements, the 22-y half life of 210Pb is used as an advantage. 210Pb is often considered to be relatively immobile in glass after alpha recoil implanted by 222Rn progenies. The diffusion of 210Pb could, however, lead to uncertain wrong retrospective radon exposure estimations if 210Pb is mobile and can escape from glass, or lost as a result of cleaning-induced surface modification. This diffusion was studied by a radiotracer technique, where 209Pb was used as a tracer in a glass matrix for which the elemental composition is known. Using the ion guide isotope separator on-line technique, the 209Pb atoms were implanted into the glass with an energy of 39 keV. The diffusion profiles and the diffusion coefficients were determined after annealing at 470-620°C and serial sectioning by ion sputtering. In addition, the effect of surface cleaning on diffusion was tested. From the Arrhenius fit, the activation enthalpy (H) was determined, which is equal to 3.2 ± 0.2 eV, and also the pre-exponential factor D 0, in the order of 20 m2s1. This result confirms the assumption that over a time period of 50 y 209Pb (and 210Pb) is effectively immobile in the glass. The boundary condition obtained from the measurements had the characteristic of a sink, implying loss of 209Pb in the topmost surface at high temperatures.