Molecules chemisorbed on a platinum single crystal are desorbed by a pulsed laser, ionized by an electron beam or multiphoton ionization, and detected by Fouriertransform mass spectrometry (F.t.m.6.). Laser desorption of ethylene, methanol, cyanogen, benzene and naphthalene is described. In all cases, molecular ions are the major peaks observed in the mass spectra, and the minor peaks correspond to known electronimpact fragments of the adsorbates. For the systems investigated thus far, the laserdesorption F.t. mass spectra are less complex and easier to interpret than the spectra produced by secondary-ion mass spectrometry, which are complicated by the rapid ionlmolecule reactions that can occur directly above the surface between adsorbates and substrate atoms. In the laser-desorption method, these complications are avoided because the ions are formed after the desorbed molecules have moved away from the surface and have expanded into the vacuum. The sensitivity of the laserdesorption F.t.m.s. method is tested. For carbon monoxide adsorbed on platinum, the detection limit is ca. 5 X 10e6 monolayer per cm2. For naphthalene, a single laser pulse at 248 nm produces abundant molecular ions even when the electron beam is turned off. The ions appear to be produced by resonance-enhanced multiphoton ionization rather than a thermal process. In these experiments, multiphoton ionization of naphthalene at 248 nm is about 35-fold more sensitive than electron ionization.