A spectrometer was used in the laboratory to study water-vapor isotope ratio measurements in air: H2 18O/H2 16O and HDO/H2 16O near 6.7 microm. The spectral region ranging from 1483 to 1487 cm(-1), which is suitable for the in situ laser sensing of major water-vapor isotopologues in the middle atmosphere from airborne or balloonborne platforms, was investigated by use of a continuous-wave distributed feedback quantum-cascade laser. The concentrations obtained were compared with the concentrations obtained with a hygrometer. The sigma(18O) values were found to be in excellent agreement with the standard value for two individual lines. The sigma(D) value was slightly higher than the standard value.