A bench-scale plant for (advanced) biological P elimination was operated with mechanically pretreated domestic sewage. It consisted of an anaerobic and an aerobic tank and a clarifier. The plant was operated with and without addition of precipitants (3 and 6 mg/l Fe). The polyphosphates were examined by nuclear-magnetic resonance spectrometry of aerobic activated sludge, in some cases after pretreatment with NaOH, and by X-ray spectrometry. The polyphosphates were associated either with Ca or with Mg and K. This became evident from the X-ray spectra of polyphosphate granules. The Mg and K was released in the anaerobic and taken up in the aerobic tank parallel to the phosphorus. The Ca on the other hand, remained immobilized. Obviously it was metabolically inactive. The addition of 3 mg/l Fe did not affect the biological P removal, and the time required for a complete P removal in the aerobic tank was even shorter than the time without addition of Fe.