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The behaviour of studied samples in electric field at microwave frequency is characterised by the real and imaginary part of the complex dielectric constant. Aknowledge of these parameters permits to predict the running of the microwave heating. The article describes the influence of temperature on the change of real and imaginary permittivity of siderite, whose grain size is 0,315 – 0,5 mm, at frequencies of 615 MHz, 1413 MHz and 2216 MHz. The permittivity was measured by the resonant cavity method. Strong changes of permittivity were detected at the temperature around 500 °C. The influence of thermal changes of permittivity on the microwave heating of siderite is analysed.