Öz Under greenhouse conditions a pot experiment was carried out to investigate the comparative interaction of Cs and K in the shoots of plants, and to interpret the way that K+ ions related to that of Cs+ in soil-plant systems. Also the soil properties that affect Cs uptake by Trifolium pratense plants were determined. The plants grown on four different soil types, contaminated with 40 mg kg-1 of Cs, and sown at 60, 240 and 420 days after contamination (treatments). During each treatment two cuts of the shoots were performed. According to the results significantly variations on Cs uptake by Trifolium pratense plants observed among different cuts, soil types and treatments. Regardless to soil type or treatment, it was found that as the concentration of Cs decreased, the K content increased in shoots in both plant cuts. Such relationship, confirms the occurrence of a direct competition between Cs and K ions during the process of accumulation through the plant tissues, suggesting that the two elements could have a common accumulation mechanism and that Cs uptake by plants can be suppressed by the competition of Cs and K ions in the plant tissues. Due to the competitive interactions that occurred between Cs and K ions, the discrimination factor (DF), which is often used to evaluate plant's efficiency to absorb nutrients from soil, was additionally estimated. In all treatments DF values were below unity, suggesting a preferential uptake of K over Cs. Thus, K appears to be one of the main factors influencing the plant mobility of Cs and behaves not only as a competitor to Cs ions, but also as an effective inhibitor of Cs uptake by plants. Among the soil properties it was observed that the particle size fractions as well as the K content in soils play a predominant role on Cs availability to plants. Thus, Cs uptake by plants is a result of reactions both in the soil and in the plant, implying that Cs transport from soil to plant is controlled by a complex mechanism. Therefore the utilization of the K-status of plant tissues simultaneously with the soil properties is essential to monitor and estimate soil to plant Cs mobility.