The optical constants of solid carbon dioxide are needed, for example, to accurately calculate the energy balance of the seasonal polar caps of Mars, which are composed primarily of CO2 ice. Only a few measurements of uncertain accuracy have been made in the broad wavelength regions of weak absorption between the isolated narrow infrared absorption bands of solid CO2, which determine the reflectance and emissivity of these polar caps. A laboratory experiment was therefore undertaken to improve on and extend the wavelength range of the previous data by measuring transmission through thick samples of high quality to determine the spectral absorption coefficient α in infrared wavelength regions of low absorption between 1.8 and 333 μm (30--5555 cm-1 in wavenumber). A technique was developed to grow clear, thick CO2 ice samples from the gas at a temperature of 150 K, with optical path lengths between 1.6 and 107.5 mm. The extremely fine spectral structure of this material was measured using resolutions finer than 0.5 cm-1. The transmission data for all thicknesses are combined in a minimizing routine that estimates both the scattering losses for each sample (typically <20% extinction) and α (between ~0.1 to 4000 m-1) at each wavelength, on the assumption that the scattering varies slowly with wavelength. The accuracy of the derived absorption coefficients was also estimated and is best in regions where α is not near the limits of measurement. Absorption coefficients obtained from the new measurements are an order of magnitude or more lower than earlier estimates in the transparent wavelength regions between the strong infrared bands. Also, many weak bands between 1.8 and 5.2 μm, which were only poorly defined previously, have been more accurately measured.¿ 1997 American Geophysical Union