A new desorption/ionization mass spectrometric technique, K+ ionization of desorbed species (K+IDS), is evaluated as a rapid method for differentiating various organic acidemias, conditions in which excessive levels of organic acid metabolites are present in plasma or urine as a result of some inborn error of metabolism. This method requires no derivatization of the isolated organic acids, unlike that required for gas chromatographic and gas chromatographic/mass spectrometric analyses. 'Batch' mass spectrometric analysis is achieved by deposition of the complex organic acid mixture (from urine) onto a K+IDS probe. Rapid heating results in the emission of alkali ions (Na+ or K+) from a thermionic emitter and the intact desorption of analyte. Subsequent gas-phase addition produces a mass spectrum showing alkali ion adducts of the components, providing molecular weight and relative concentration information. This rapid desorption/ionization technique requires no matrix, and analysis times are exceedingly short relative to those required in gas chromatographic/mass spectrometric analyses. Results suggest that differential diagnosis of some of the more commonly occurring organic acidemias (e.g. isovaleric acidemia, maple syrup urine disease, etc.) may be made.