We report the preliminary results of experimental studies of a dense cesium vapor in all-sapphire sealed-off Cs cell (temperature up to 500 °C, atomic and molecular densities up to 1018 and 3.2x1016 cm-3, respectively) in selective reflection (SR) configuration. Probing the vapor by a 780 nm laser radiation scanned in 20 GHz range, we have observed a spectral structure, which appears in the vapor temperature range of 250 - 400 °C. Though the cell is filled with pure cesium, starting from 300 °C we have revealed a noticeable SR contribution of rubidium atomic D2 line transitions that fall in the laser tuning range. The spectral features observed in present work can be attributed not only to cesium dimers (Cs2) having absorption band in this spectral region, but also to RbCs molecules. Relevant spectroscopic properties of these molecules are also addressed. We believe this is the first observation of selective reflection from the interface of dielectric and molecular vapor. More experiments and theoretical treatment is required to clarify the mechanisms involved and identify the origin of each spectral component, the further work program is outlined.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.