The freezing properties of sulfuric acid droplets were studied by suspending single 20- to 30-μm-diameter particles in a quadrupole trap and cooling them to stratospheric temperatures (≥191.5 K). Each particle's dc balance voltage was measured to determine the particle composition as a function of temperature and map out the particle's trajectory relative to the sulfuric acid phase diagram. Angularly resolved optical scattering patterns were monitored to detect freezing events. Particles cooled through the sulfuric acid tetrahydrate region (35--70 wt % H2SO4) did not freeze and remained spherical liquid droplets for several hours. Only particles cooled through the ice-liquid equilibrium region (<35 wt % H2SO4) showed evidence of freezing. This supports previous experimental and field observations that stratospheric sulfuric acid aerosols are likely to remain liquid to within a few degrees of the ice frost point.¿ 1997 American Geophysical Union