In order to understand the behaviour of radioactive and stable iodine in the environment, we have carried out radiotracer experiments and chemical analyses in the soil-plant systems. Parameters important for the assessment of radioiodine movement from the environment to man, e.g. soil-plant transfer factors for various agricultural crops and soil-solution distribution coefficient for different soils, were obtained. Mechanisms of iodine sorption and desorption on soil were also studied. Microorganisms and/or their products (e.g. enzymes) were found to play an important role in the fixation of iodine on soil. Iodine was observed to be desorbed from the flooded soils due to the reducing conditions (low Eh) created by the microbial activities. From the soil-rice plant system biogenesis methyl iodide was found to be evaporated into the atmosphere. Through experiment using 1Z3 I tracer, we found that volatile organic iodine was produced due to microbial activities (including bacterial activities).