Microwave assisted extraction enhances antioxidant extraction kinetics in grape marc processing. For an effective microwave process design, the dielectric properties (DPs) of the material must be known under different conditions. This study focused on the DP measurement of grape marc using a resonant cavity measuring device. Grape skin, seeds and their mixtures with three different moisture contents were studied at 28, 39 and 50 degrees C. At higher moisture content DP showed increasing tendency, while temperature didn't have a definite effect. Good correlation was achieved with the experimental results applying the complex refractive index mixing equation between 0.4 and 0.6 porosity. Also, despite the complexity of natural products properties, the experimental DP values of the real mixture were in agreement with values estimated from DP properties of its individual constituents (grape skin and seeds). Milling was performed to obtain homogeneous material for the measurements, and this effect on the results was also studied. (C) 2013 Elsevier Ltd. All rights reserved.