[en] The caesium plate-out on Incoloy 800 and the Ni-base alloy Inconel 617 was investigated. Deposition experiments have been performed in a high temperature helium circuit using a vapour pressure controlled caesium source. The experiments were carried out for 20-60 days at temperatures between 600 and 900 deg. C under oxidizing conditions. The following values were obtained for Incoloy 800 at 600-800 deg. C: desorption energy: 234 ± 13 kJ/mole; volume sorption probability: 1.10^-6 - 2.10^-5. For Inconel 617 a desorption energy of 230 ± 8 kJ/mole at 700 deg. C with a linear increase to 268 ± 8 kJ/mole at 900 deg. C was determined. The preexponential factor of the desorption constant was set to 1.10¹⁴ s^-1. The experimental results are influenced by the in-situ build-up of oxide layers. The plate-out parameters of Incoloy 800 have been applied to a long-term experiment in the Dragon reactor (480 days at 680 deg. C). The calculated results agree very well with the measurements. Furthermore, the caesium deposition in an HTR primary circuit has been calculated in order to assess the sensitivity of the above uncertainty ranges. Due to these uncertainties, the upper limits of the calculated surface activities in the primary circuit are about a factor 2 above the expected values, which is acceptable for design purposes. (author)