The heating behavior of a food product in a radio frequency (RF) heater with a free-running oscillator largely depends on the dielectric properties of the food material in processing. In this study, heating rate was mathematically derived as a function of its influencing factors in a RF system. This relationship was validated by experiments using conditioned salt solutions and peanut butter samples in a 27.12 MHz, 6 kW RF system. The dielectric properties of the materials used to validate the model ranged from 3.3 to 91.6 for dielectric constant and from 0.1 to 1577.0 for dielectric loss factor. The comparison between theoretical and experimental results showed a good agreement for the tested samples. Both dielectric constant and loss factor influenced the heating rate under a fixed electrode gap and frequency. When the values of dielectric constant and loss factor were close to one another, the maximum heating rate can be reached. (C) 2013 Elsevier Ltd. All rights reserved.