We have determined the infrared optical constants of sulfuric acid solutions at intervals of approximately 5 wt% between 45 and 80 wt% H2SO4 at a temperature near 215 K. The optical constants were determined by first measuring the transmission of infrared light (7000 to 500 cm-1) through sulfuric acid films of varying thickness. An iterative Kramers-Kronig technique was then used to determine the optical constants that yielded the best match between calculated and measured spectra. Finally, a fitting routine was used to create a method for interpolating between the sulfuric acid compositions where optical constants were measured. The optical constants determined here differ considerably from those measured at warmer temperatures using reflection techniques. For some frequencies and compositions, the imaginary refractive index is more than twice as large as was previously measured at 300 K. The change in optical constants has significant implications for retrievals of aerosol mass and composition from satellite measurements. ¿ 1998 American Geophysical Union