Polyimide-based materials, like Kapton, are widely used in flexible cables and circuitry due to their unique electrical and mechanical characteristics. This study is aimed at investigating the radiopurity of Kapton for use in ultralow background, rare-event physics applications by measuring the 238 U, 232 Th, and nat K levels using inductively coupled plasma mass spectrometry. Commercial-off-the-shelf Kapton varieties, measured at approximately 950 and 120 pg/g 238 U and 232 Th (1.2×10 4 and 490 µBq/kg), respectively, can be a significant background source for many current and next-generation ultralow background detectors. This study has found that the dominant contamination is due to the use of dicalcium phosphate (DCP), a nonessential slip additive added during manufacturing. Alternative Kapton materials were obtained that did not contain DCP and were determined to be significantly more radiopure than the commerciallyavailable options with 12 and 19 pg/g 238 U and 232 Th (150 and 77 µBq/kg), respectively. The lowest radioactivity version produced (Kapton ELJ, which contains an adhesive) was found to contain single digit pg/g levels of 238 U and 232 Th, two-to-three orders of magnitude cleaner than commercial-off-theshelf options. Moreover, copper-clad polyimide laminates employing Kapton ELJ as the insulator were obtained and shown to be very radiopure at 8.6 and 22 pg/g 238 U and 232 Th (110 and 89 µBq/kg), respectively.