Microwave assisted chemistry has become a popular research topic within the framework of process intensification. The physics of microwave heating are fundamentally different from those found in conventional processing systems. Heating occurs fast, but non-uniformly in hard-to-predict patterns. In this study it is shown that these complexities are caused by the resonant nature of the microwave fields present in heating devices. This is demonstrated by means of experiments and modeling concerning heating of water-filled vials in single mode resonant microwave equipment. The results evince highly non-uniform, transient and irregularly behaving heating processes. (C) 2012 Elsevier Ltd. All rights reserved.