On 25 January 1998 mountain induced gravity waves developed over Scandinavia leading to the formation of mesoscale polar stratospheric clouds (PSCs). Ballon-borne mass spectrometric measurements of particle composition and optical backscatter measurements were performed above Kiruna/Sweden. PSCs were encountered twice, showing a correlated increase in the condensed phase water, nitric acid and the backscatter ratio. Thermodynamic modeling allows the PSC particles to be unambiguously identified as supercooled ternary solution (STS) droplets, but cannot account for the measured scatter in the particulate HNO3:H2O mole ratio. Simultaneous temperature measurements show that the particles were subject to rapid atmospheric temperature fluctuations of ¿1 K and cooling/heating rates exceeding 1 K/min caused by the gravity waves. Micro-physical non-equilibrium modeling of STS droplet distributions reveals that the observed temperature perturbations cause particle compositions in close agreement with the measured HNO3:H2O variations. Non-equilibrium compositions of liquid PSC particles are thus a principal stratospheric characteristic related to gravity waves affecting particle evolution. ¿ 2000 American Geophysical Union