The stability of submicron powders of geothite (α-FeOOH) and lepidocrocite (&ggr;-FeOOH) to dehydration by ultraviolet (UV) radiation was investigated by laboratory experiments. Dehydration of these minerals by solar UV radiation is potentially an important process on the surface of Mars. When the spectral irradiance of the incident radiation was about 100 to 300 times that of the solar spectral irradiance at 1.5 AU (the orbit of Mars), radiant heating was so intense that both FeOOH polymorphs were thermally dehydrated. When the above incident radiation was optically filtered to remove a large portion of infrared and visible components, but not the intense near-UV component, no perceptible dehydration, and therefore no UV photodehydration, was observed after irradiation times as long as 420 hours. After scaling the laboratory irradiance to that at the Martian surface, we calculate that goethite, lepidocrocite, and presumably all FeOOH polymorphs would not perceptibly UV photodehydrate there for at least 10 to 100 years. That is, there is as yet no basis in laboratory experiments for inferring that UV photodehydration of FeOOH polymorphs will occur naturally on the saurface of Mars on a time scale of 10 to 100 years, if at all. Thus, the observation that goethite may contribute to the spectral reflectance data of Mars (e.g., Singer et al., 1979) is consonant with out results. The formation of maghemite (&ggr;-Fe2O3) by UV photodehydration of lepidocrocite does not seem a viable mechanism for producing the former to account for the magnetic component apparently present in Martian surficial material (Fuller and Hargraves, 1978; Hargraves et al., 1979). Finally, ferrimagnetic Δ-FeOOH remains a viable candidate (Burns, 1980) for the magnetic material with respect to FeOOH stability to UV photodehydration.