A neutron activation analysis method with pre-irradiation separation and post-irradiation purification is presented for the determination of ¹²⁹I content and ¹²⁹I/¹²⁷I ratio in some environmental samples, such as seawater, seaweed, grass and soil. An evaporation concentration followed by solvent extraction was used for the separation of iodine from seawater, and alkaline ashing or alkaline fusion followed by water leaching and solvent extraction for vegetation and soil samples. A further post-irradiation purification of iodine was carried out by extraction and PdI₂ precipitation. The concentrations of ¹²⁹I and ¹²⁷I were then measured by counting ¹³⁰I and ¹²⁶I, respectively. The recoveries of iodine reach 60–95% and more than 98% in pre-concentration and post-irradiation purification procedures, respectively. The detection limit of this method for ¹²⁹I reached 2–3 × 10^–13 g. Some samples collected in Denmark and Iceland were analysed.