The adsorption behavior of n-pentane and cyclohexane in mica slits at room temperature has been studied as a function of chemical potential and gap width with multiple-beam interferometry. The measured film thicknesses close to saturation for large slit widths (effectively isolated surfaces) range up to 7 nm with n-pentane (at a relative vapor pressure of 0.9996) and 3 nm with cyclohexane (at a relative vapor pressure of 0.995). The thickness of these adsorbed wetting films is slightly larger than that predicted by van der Waals theory. The difference may be accounted for by thermal fluctuations of the adsorbed liquid-vapor interface. At smaller slit widths a capillary condensation transition occurs as the slit fills up with liquid. The separation at which this occurs is in good agreement with a film-thickening mechanism due to van der Waals forces across the gap only for the thickest films (t greater than or equal to 6 nm). For thinner films the capillary condensation transition occurs at larger than expected slit widths, and the deviations are large for t less than or equal to 3 nm, We speculate that these larger than-expected condensation separations are related to a fluctuation-enhanced film thickness in this regime. The work demonstrates the utility of measurements in a system consisting of a single slit-pore, without the complications of polydispersity and connectivity of pore networks. The results show that vapor adsorption isotherms can be measured with multiple-beam interferometry, i.e., in the surface force apparatus.