The discrimination of optical isomers (enantiomers) in the gas phase has been performed using thickness shear mode resonators (TSMRs). The selective coatings included both enantiomers ((S)- and (R)-receptor) of a Chirasil-Val(R)-derivative (stationary phase material in gas chromatography (GC)) with octyl-side chains. A successful discrimination of the enantiomers of different types of analytes (amino acids and lactates) was achieved. The results were in good agreement with GC-measurements. In addition different mixtures of the enantiomers of the respective analyte were investigated and the enantiomeric composition could be determined quantitatively with excellent reliability. Thus, the sensors allow an on-line-monitoring (not possible for GC) of enantiomeric purity.