The detection of C60 and C70 fullerenes in young planetary nebulae and in reflection nebulae suggests that these molecules are more common in certain astrophysical environments than previously thought. The dependence on temperature of the positions and widths of the infrared bands of the C60 and C70 fullerenes is needed for a firm qualitative detection of these molecules in space. Furthermore, the integrated molar absorptivity Ψ (in km mol-1) of each infrared absorption band is required for a quantitative determination of the abundance of C60 and C70 in space. In this paper, we report on the temperature dependence of the wavelength shift and integrated molar absorptivity of the infrared bands of the C60 and C70 fullerenes. The measurements have been made in a KBr matrix in the temperature range between -180°C and +250°C. The experimental data have been extrapolated to derive both the infrared band shift and the integrated molar absorptivity of the C60 and C70 fullerenes at absolute zero temperature.