[en] Introduction and Objective: The temperatures inside the pebble-bed reactor where the fuel spheres are continuously moving from the top the bottom are not directly measurable with usual technique. A measure for the average temperature of the AVR primary coolant gas leaving the core is determined using measurements on the power of the steam generator, the temperature of the coolant gas fed back to the core and the coolant flow through the core. The aim of this experiment was to determine the maximum gas temperatures for the primary gas in the core and the radial distribution of these temperatures. For this reason special prepared monitor spheres were inserted into the core monitoring the maximum gas temperatures. After their passage through the core the elements were separated and examined.