Release data of cesium in Biso low-temperature isotropic (LTI) and high-temperature isotropic (HTI) coated particles are presented. Experimental and theoretical methods are briefly discussed. From the analysis of our experiments, kernel diffusion coefficients are calculated covering the kernel burnup range from 0.2 to 16% FIMA. At FIMA values ≥5%, cesium release is governed by the relatively low activation energy of ∼19 kcal/mole (79.6 kJ/mole), and even at temperatures as low as ∼900°C (1173 K), kernel retention remains low. The outer pyrocarbon layer acts as the release rate controlling barrier for both LTI and HTI coatings. Diffusion coefficients in the temperature range from 1000 to 1500°C (1273 to 1773 K) for LTI coatings and 1250 to 1600°C (1523 to 1873 K) for HTI coatings have been derived and lead to the following Arrhenius equations: On the basis of these data, releases for high-temperature-reactor cores can be calculated.