31P-NMR has been tested as a technique for determining the phosphorus content of milk samples in both the inorganic form (Pi) and bonded to caseins as phosphoserine (SerP). 31P-NMR made possible the simultaneous determination of Pi and SerP in ∼ 1·7 h. The determination is based on measuring the area under the resonance in question and interpolating in a calibration curve prepared using external standards of known phosphorus contents and prepared in the same manner. The concentration–area plots obtained were linear in the ranges 162–1993 mg P/l for Pi and 38–402 mg P/l for casein SerP. The repeatability of the method was good. When the same milk samples were analysed by both 31P-NMR and the classic colorimetric method the correlation was good. Polyphosphates added to commercial milk were also detected, as diphosphates. The NMR response for diphosphate was linear between 58 and 576 mg P/l and preliminary results indicated that quantitative analysis could be achieved in commercial milk.