Results are presented for the radiation‐induced transient darkening of thin organic polymer films normally used as Cerenkov light emissions sources. The radiation source is a 27‐MeV, 10‐μC, 200‐ns electron beam generated by the PHERMEX accelerator. The typical dose for a single pulse is 5 Mrad. At this dose, the broadband time‐resolved percent transmission above 520 nm was measured for four common polymers: polyimide (Kapton‐H), polyethylene terephthalate (Mylar), cellulose acetate, and high‐density polyethylene. Kapton was found to darken the most and polyethylene darkened the least. The recovery time to normal transmission for Kapton was found to be greater than 10–20 μs. The radiation‐induced attenuation coefficient is shown to depend on electronic band energy separation. The results show that Kapton is not the material of choice for a Cerenkov light source.