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In this work the results of the experimental study of surface tension (ST) of alkali metals and their Na-K-Cs ternary system over the whole composition range and for temperatures from the melting point to 680 K are presented. Careful thermal treatment for several hours at 700 K under 10−7 Pa vacuum condition and a new way of loading the measuring cell have been used to exclude possible contamination of the liquid metals and the measuring cell. To measure ST the sessile-drop method, which yields an experimental error of 1% with 95% probability, was used. Contents of impurities were not larger than 0.005%.Surface tensions of 110 Na-Cs-K alloys have been measured over the whole composition triangles for the first time. Polytherms of ST for pure metals and ternary alloys are well described by linear equations. The isotherms of ST for each Na/Cs + K section have been constructed. It was found that K is a surface-active addition in ternary alloys when XNa:XCs > 14:1, while it becomes a surface-inactive component if XNa:XCs < 14:1. All ternary alloys along the XNa:XCs = 14:1 section have the same ST, independent from K content. Lines of equal values of ST have been plotted for the whole composition triangles as well.