The results of measurements of the refractive index and power attenuation coefficient of Zitex at 290, 77, and 4 K in the spectral region from 1 to 1000 μm are presented. Zitex is a porous Teflon sheet with a filling factor of ∼50% and is manufactured in several varieties as a filter paper. Zitex is found to be an effective IR block, with thin (200-μm) sheets transmitting less than 1% in the 1–50-μm range while attenuating ≲10% at wavelengths longer than 200 μm. Some variation in the cutoff wavelength is seen, tending to be a shorter-wavelength cutoff for a smaller pore size. In addition, the thermal conductivity of Zitex at cryogenic temperatures has been measured and is found to be roughly one half that of bulk Teflon. Finally, its dielectric constant has been measured in the submillimeter as n = 1.20, resulting in extremely low dielectric reflection losses. As a result, Zitex is particularly useful as an IR blocking filter in low-noise heterodyne receivers; in the millimeter-wave range (λ ≳ 850 μm or ν ≤ 350 GHz) the attenuation of α ≤ 0.01 cm^-1 for a 3.5-mm thickness filter of Zitex G125 would raise receiver noise temperatures by <1 K.