31P-NMR spectra were obtained from adenosine-5'-tetraphosphate under a variety of conditions and the four discernible resonances assigned. The pK-value of the metal-free compound was determined to be 6.4, the pK-value of the Mg2+ complex to be 5.3. The dissociation constant for the AP4 X Mg2+ complex was estimated to be 10(-4) M from the downfield shift of the resonances assigned to the gamma- and delta-phosphorus nuclei. The binding of a second metal ion can also be followed by NMR; the dissociation constant for this ternary complex is several orders of magnitude larger than that for the binary complex.