Earlier work on the coprecipitation of230Th and234Th with BaSO4 has been confirmed and in particular the influence of other ions was investigated. Potassium ions, both in the solution and in the crystals, promote the coprecipitation, while polyvalent cations tend to prevent it. It is shown that increased concentration of K+ causes a marked change in the BaSO4 crystal form, with a concomitant increase in surface area. At the same time, the rate of exchange of133Ba2+ with the BaSO4 surface is also increased. This can be roughly accounted for by the change in surface area. It is conjectured that an equilibrium occurs involving foreign cations in the lattice.