Information on the behavior of the dielectric properties of electrolytic solutions, such as reaction mixtures, during acid-catalyzed transesterification reactions in biodiesel production is essential in the design of processes involving microwave heating technology. The influence of the acid catalyst on the dielectric parameters during the methanolysis and ethanolysis of crude Brazil nut oil was investigated employing H2SO4, H3PO4, ClSO3H, and CH3SO3H as the catalysts. Refined soybean, refined babassu, and waste Brazil nut oils were used to study the dielectric behavior during the transesterification reaction catalyzed by H2SO4. Measurements were taken using a vector network analyzer with an open-ended coaxial probe (from 300 MHz to 13 GHz). The dielectric properties of the reaction mixtures during the transesterification reaction were frequency dependent and varied markedly with the conversion to biodiesel. The catalyst used significantly affected the dielectric properties, contributing to the high values (0.14-0.40) for the loss tangent of the reaction mixtures. The high content of free fatty acids and water in the oils also contributed considerably to the dielectric behavior during the transesterification reaction.