The physical properties of supercooled sulfuric acid solutions are of interest because sulfate aerosols play a role in heterogeneous stratospheric chemistry and possibly in the nucleation of polar stratospheric cloud particles. We report measurements of the viscosity of sulfuric acid solutions at temperatures between 200 and 300 K and for concentrations between 30 and 80 wt %. These results can be used to estimate diffusion constants and to calculate nucleation rates in sulfuric acid at low temperatures. The current measurements give higher viscosities at lower temperatures than previous approximations. This in turn decreases estimated diffusion constants and nucleation rates.