A new method for inorganic phosphate microquantification is introduced based on negative ion electrospray tandem mass spectrometry and stable isotope dilution by (18)O4-labeled phosphate. Quantification is performed using the non-labeled and (18)O3-labeled [P(18)O3](-) fragment ions at m/z 79 and 85, respectively, formed by dissociation of the [H2PO4](-) ion at m/z 97 and 105, respectively, visible in negative ion electrospray ionization mass spectrometry (ESI-MS) spectra. Tandem mass spectrometry was selected to remove an overlap with the isobaric [HSO4](-) ion at m/z 97 of sulfate and to establish an optimal sensitivity of the quantification assay. It is demonstrated that the assay can also measure the sum of inorganic and phosphoryl phosphate by prior enzymatic hydrolysis of phosphoryl phosphate. The assay works with phosphate concentrations in the micromolar range and, in combination with nano-ESI, is capable to quantitate absolute amounts of phosphate in the low nanogram range from complex samples.