Fourier transform infrared spectroscopy data are reported concerning the adsorption of formaldehyde, acetaldehyde, acetone, cyclopentanone, cyclohexanone, and 2-cyclohexen-1-one on Aerosil outgassed at 800 degrees C. With the four heavier species, plain physisorption competes with the formation of H-bonded complexes with the isolated surface hydroxyl, which are best studied at vanishing coverage. The stretching mode of H-bonded hydroxyls shows in all cases an unusually complex shape (changing when physisorption takes place), because of the coupling between the OH stretch and the strongly anharmonic intermolecular modes. From the intensity of the nu(OH) mode of the free hydroxyls as a function of pressure, the equilibrium constant for the H-bond formation is calculated. Several correlations are drawn for Delta nu(OH), Delta nu(CO), and Delta G degrees. An estimate for Delta H degrees is made from Delta nu(OH), SO that a fairly complete thermodynamic characterization of the H-bonding process is achieved.