The design of processes involving microwave technology is dependent on the dielectric properties of the materials being processed. The dielectric constant and loss factor of the pure liquids methanol, ethanol, glycerin and sulfuric acid, as well as vegetable oils (Brazil nut and soybean), were measured using a vector network analyzer in an open-ended coaxial probe method (frequency range 300 MHz to 13 GHz) at various temperatures. For measurement during acid-catalyzed transesterification two oil: ethanol molar ratios at three different acid concentrations were employed. Dielectric properties of the pure samples and reaction mixtures during the transesterification reaction were frequency and temperature-dependent. The catalyst significantly affects the dielectric properties mainly at high alcohol concentrations where the mobility of counterions and the loss factor are high. The results indicate that the influence of the catalyst is reduced at low alcohol concentrations as the mobility of the counterions becomes more restricted decreasing the ionic conductivity. (C) 2013 Elsevier Ltd. All rights reserved.