Although microwave enhanced chemistry has become an established research topic, the actual electromagnetic interactions have received little attention. Following a previous study on the spatial distribution of heat generation, this study addresses the high sensitivity of the overall heating rate to geometrical and medium parametric variations in a highly celebrated microwave heating device (CEM Discover), which serves as a representative example of resonant microwave systems. It is demonstrated, both by experiment and simulation, that this high sensitivity hampers the prediction of total heat generation. Consequently, in order to optimize process performance, precise information regarding all relevant geometrical and medium parameters of the coupled microwave equipment-reactor system should be taken into account. (C) 2012 Elsevier Ltd. All rights reserved.