We have observed at high spectral resolution (R similar to 50000) the 4.6 mu m region of binary post-AGB objects with circumbinary dust disks. Prototype objects are HR 4049 and the Red Rectangle. For most objects our infrared spectra reveal the presence of CO vibration-rotation lines and in some objects H2O vibration-rotation lines. The most extreme of these objects is HR 4049 where the 4.6 mu m spectrum shows a rich forest of CO and H2O emission lines of circumbinary origin. The presence of H2O lines confirm the oxygen-rich nature of the circumbinary gas which is in contrast to previously detected carbon-rich dust in many of these objects. Modeling of the line profiles suggests that the circumbinary gas is located in a thin layer near the dust disk. Modeling the circumbinary line profiles allows the masses of the individual stars in the binary to be derived. In the cases analyzed so far the supergiant has mass typical for a white dwarf confirming its post-AGB nature. The orbital parameters of these stars require a previous stage of common envelope evolution suggesting an origin of the circumbinary disk in the common envelope stage.