Coke formation affects the activity of catalysts and the product spectra in refinery and petrochemical processes. In previous works, it was shown that the coke content in heterogeneous catalysts can be monitored in-situ by measuring the electrical impedance of single catalyst pellets. In this initial study it should be clarified whether in principle it is possible to apply a contactless microwave-based measuring method to quantitatively detect coke in heterogeneous catalysts. The advantage would be that a contactless method is more suitable for an industrial application since only one electrically insulated feed-through is necessary to determine the coke load in situ in a reactor with its rough reaction conditions (up to 500 degrees C, 30-50 bar, high H-2 partial pressure). (C) 2011 Elsevier B.V. All rights reserved.