Three-dimensional magnetic resonance imaging (MRI) was used to visualise polyurethane foam-immobilised Citrobacter after challenging with La3+ and/or Cu2+ in citrate buffer supplemented with glycerol 2-phosphate. Extensive phosphatase-mediated bioaccumulation of LaPO4 was observed but no evidence for deposition of Cu-3(PO4)(2) was obtained by X-ray diffraction and proton-induced X-ray emission analyses. Image analysis showed that La3+/Cu2+ is a good model system to study the function of this biofilm reactor non-invasively by MRI.