We report a new microcalorimeter for measuring heats of adsorption of low vapor pressure molecules on clean single crystals. While temperature programmed desorption (TPD) and isosteric measurements can provide information on heats of adsorption, they are limited to reversible adsorption-desorption processes. In many catalytically interesting cases, adsorbates undergo irreversible chemical changes upon adsorption or heating. Our microcalorimeter enables direct measurement of these adsorption energies. The principle is similar to that pioneered by King's group:@footnote 1@ a pulse of gas from a molecular beam impinges on a 1 µm-thick single crystal surface. The incident molecules adsorb either physically or chemically, causing a transient temperature rise. This heat input is detected by a 9 µm thick pyroelectric polymer ribbon, which is mechanically driven to make gentle contact with the back of the single crystal sample during a calorimetry measurement.@footnote 2@ This process allows the determination of heats of adsorption as a function of coverage with femtomole resolution. While King's group has performed beautiful adsorption calorimetry, their measurements have been limited to high vapor pressure gases. We have added a specially designed molecular beam source for low vapor pressure molecules, thus significantly extending the capabilities of this technique. Sticking probabilities and absolute coverages are measured by quadrupole mass spectrometry (QMS), low energy ion scattering (LEIS), x-ray photoelectron spectroscopy (XPS), and TPD. As a first experiment, we present a microcalorimetric measurement of benzene adsorption on Pt(111). @FootnoteText@ @footnote 1@ W.A. Brown, R. Kose, and D.A. King, Chem. Rev. 98, 797 (1998). @footnote 2@ J.T. Stuckless, N.A. Frei, and C.T. Campbell, Rev. Sci. Instruments, 69, 2427 (1998).