Microwave assisted extraction has been proved to be a useful tool for the recovery of valuable compounds from natural products. However, little is known about the dielectric properties for mixtures of natural products and solvents. In this work, a semi-empirical model is proposed to calculate the dielectric properties of these systems. The variables considered were temperature, solvent composition and solid liquid ratio. Grape pomace was used as the solid matrix and ethanol-water mixtures as the solvent. The experimental dielectric constant and loss factor of these multiphase solid-solvent samples were measured and compared to the proposed model. Loss factor values were taken under special consideration, since they were boosted by the release of ions from the solid (an increase between 9 and 83%). The model proposed here reproduces the dielectric properties of this multiphase media with an average deviation of 8.1% for the dielectric constant and of 24.8% for the loss factor. (C) 2016 Elsevier Ltd. All rights reserved.