Electromagnetic energy in the microwave and radiofrequency (RF) portions of the spectrum can be used to heat or defrost (thaw) foodstuffs. This paper considers a fixed frequency RF oven, where a matching circuit is used to ensure that the combined impedance of the oven and the food (the load) matches the impedance of the generator and the transmission line. A controller measures the reflected RF signal and adjusts the settings of components in the matching circuit. The system is highly nonlinear and there are significant sources of uncertainty in the model of the process. A variable structure controller is designed and its stability is examined in the presence of uncertainties due to stray components. The switching lines are defined in terms of the real and imaginary parts of the reflected signal, which avoids the need to estimate the states. Conditions are derived for maintaining stability in the region close to the matching point. The performance of the controller is illustrated by results from an implementation on a prototype commercial RF oven.