The chemical composition of polar stratospheric cloud (PSC) particles has been measured in a balloon-borne mass spectrometer experiment <Schreiner et al., 1999>. The cloud particles have simultaneously been observed by a backscatter sonde flown on the same gondola. Taken separately, the two data sets indicate that the PSC particles are composed of liquid supercooled ternary solutions (STS). The chemical analysis shows that the particles contain relatively low concentrations of nitric acid, indicating a not fully developed type 1b PSC. Assuming the particles to be composed of STS, it appears that the calculated particle volumes are too small to generate the large observed backscatter ratios. Possible explanations for the apparent discrepancy are discussed, based on STS equilibrium model and microphysical/optical model calculations and including a meteorological mesoscale analysis of the mountain lee wave situation during the flight. Assuming a relatively large value of the real part refractive index of STS particles around 1.50 at 940 nm wavelength would apparently bring the two data sets in agreement. However, this large value seems to be in conflict with previous laboratory measurements and theoretical predictions, although in better agreement with recent balloon-borne observations. The limited observational data set does not allow us to assess if the particles could be composed of a metastable dilute solid solution or a higher nitric acid hydrate. ¿ 2000 American Geophysical Union