[en] Changes in the pulse height spectra of 3H and 125I in liquid scintillation counting have been studied when scintillator solutions were doped with heavy elements such as Tl and Pb. The possibility to use this method to improve the spectra separation of 3H and 125I in double labelling experiments has been investigated. This technique with doped scintillation solutions could also be used for efficient external counting of 125I. (author)