[en] An optimized pyrolysis method was presented for release of iodine from environmental solid samples with almost quantitative recovery of iodine. The released iodine trapped in alkali solution was separated using a simple and reliable AgI precipitation method, which was directly used for determination of ¹²⁹I using accelerator mass spectrometry (AMS). The mechanism of iodine released in pyrolysis was explored. Surface soil samples collected from the Northwest China were successfully analyzed for ¹²⁹I, showing that ¹²⁹I in this area falls to the background level of North Hemisphere. (author)