VDZ, IKN and HeidelbergCement (HC) have successfully managed to cool clinker under oxyfuel technology conditions. The innovative oxyfuel clinker cooler prototype has successfully been tested in a HC plant in Hannover. The overall aim of this experimental work was the testing of an oxyfuel clinker cooler in an industrial environment in order to assess its cooling performance (efficiency), as well as the potential impacts of CO2–rich cooling gas on clinker quality. The oxyfuel clinker cooler prototype was designed with a capacity of 80 t/d. During the trials a maximum of 47 t/d clinker has been extracted, which corresponds to 1.5% (1:64 scale) of the clinker production of a reference cement plant (3000 t/d). Despite the experimental challenges, CO2 concentration levels of the cooling medium higher than 70 vol.% have been achieved repeatedly by continuous CO2 supply. No leakages of CO2–rich gas occurred during the trials, which proved the efficiency of the sealings. The testing process parameters were recorded continuously and clinker samples from the cement plant main cooler and from the oxyfuel clinker cooler were taken periodically for further analysis. The lessons learned during the trials will be a valuable input for the design of future industrial scale oxyfuel clinker coolers. The operation of the oxyfuel clinker cooler prototype indicates that boundary zones, such as the cold clinker discharge system, demand special attention regarding minimisation of false air ingress also in industrial scale projects. Moisture content of the recirculated cooling gas was much higher than expected. This was attributed to the experiment conditions and the lack of a condenser in the prototype setup. Unusual layers of up to 2 μm thickness around alite crystals in contact with pores, probably resulting from up to 4% alite decomposition, have been observed in a few analysed clinker samples. However, layer formation could not be correlated with the high CO2 concentrations in the cooling medium alone and further investigations were required. The effect of water (moisture) combined with high CO2 concentration in the cooling gas leading to a limited extent of alite decomposition was further investigated and validated. Also, the absence of negative effects of the layers on cement strength was confirmed by additional laboratory testing.