The aim of this work was to study the effects of pH, a metal complexing reagent (citrate) and the concentration of substrate on the bioaccumulation of tetravalent actinides, using Th4+ as a model, by a phosphatase-catalysed reaction. This yields HPO42- which precipitates with heavy metals as cell-bound metal phosphate. Poor removal of Th from solution was observed, which is in accordance with the solution chemistry of the metal. A considerable improvement in the efficiency of thorium removal was obtained by incorporating ammonium acetate (NH4Ac) into the solution. Although extensive deposition of polycrystalline NH4UO2PO4 was observed previously by cells that had accumulated UO22+, no evidence for deposition of crystalline thorium phosphate was obtained by X-ray diffraction analysis. Examination by proton induced X-ray emission (PIXE) analysis showed a non-homogeneous thorium deposit of variable phosphorus content.