Classical nucleation theory for homogeneous freezing as well as recent laboratory data are used to formulate an algorithm for ice nucleation from an aqueous sulfuric acid solution droplet. A new parameterized function is derived from recent thermodynamic data to express the variation of sulfuric acid solution composition with temperature and relative humidity. This function is then used to derive critical ice nucleation parameters from recent laboratory data. The critical nucleation parameters are used in a classical nucleation theory to derive the diffusion activation energy of water molecules in a sulfuric acid solution from the measurements. The derived diffusion activation energy of water molecules in a sulfuric acid solution does not agree with the diffusion activation energy calculated from a viscous flow formulation that is commonly used in classical nucleation rate calculations. Our results show that ice nucleation in a sulfate droplet occurs when the interface energy of ice against the sulfate solution is approximately 17 dyn cm-1. We calculate that a supersaturation ratio of about 1.3 to 1.5 is required to nucleate ice from an aqueous sulfuric acid solution droplet in the temperature range of about 185 to 240 K. This supersaturation corresponds to a supercooling of a sulfate solution to about 2 to 3 K below the equilibrium condensation point of ice. Simple functions are given for estimating the nucleation point of ice in the upper troposphere. The differences between previous parameterizations and this work are discussed. ¿ 1997 American Geophysical Union