[en] For various types of samples (thyroids from humans, from game and cows; ground water, snow and air filters) the ¹²⁹I-contents, expressed as ¹²⁹I/¹²⁷I-ratio (R), were determined by neutron activation analysis. The method utilises the neutron induced reactions ¹²⁹I(n, γ)¹³⁰I and ¹²⁷I(n, 2n)¹²⁶I. Four main steps are involved: 1. Separation of iodine from the sample and conversion to a form suitable for neutron irradiation. 2. Irradiation in a reactor. 3. Chemical processing of the irradiated material, i.e. separation of radio-iodine with addition of carrier. 4. Measurement of activities of ¹³⁰I and ¹²⁶I with a gammaspectrometer. The ¹²⁹I/¹²⁷I-ratio (R) is then obtained by comparing with the activities produced in a reference sample (usually with R = 1.0 x 10^-8) which is irradiated simultaneously. Details of the method are presented in the report. R-values between 0.6 x 10^-8 and 39 x 10^-8 were found. The highest values were those of snow and game thyroids. Ground water contained very little ¹²⁹I. The results confirm the expectation that ¹²⁹I enters the biosphere mainly via air and precipitation. (Author)