"Microwave effect" could be explained by the phenomenon occurring during microwave interactions with materials. In our experiments "microscopically thermal non-equilibrium by selective heating of the components" has been observed. The grains in compacted powder or molecules in a liquid absorb microwave energy selectively at microscopic level, and heat up differently depending upon the degree of absorption of respective phases. The small non-equilibrium hot spots rise, move and dissipate occasionally inside the body. The chemical reactions or phase mixing could be generated at the boundary very rapidly. It leads to rapid reactions at relatively lower temperatures. The overall bulk temperature is lower than the hot spots at microscopic level. In our experiments visible light spectroscopy was used to monitor the reactions under the microscopically thermal non-equilibrium.